From the Laboratory of Molecular Genetics, The Rockefeller
University, New York, New York 10021
The metabolic fate of covalently linked
DNA-protein complexes (cross-links) is not clearly understood. Our aim
was to investigate the processing of protein-DNA cross-links by
cellular enzymes. As an example of a DNA-protein cross-link, we have
constructed frozen topoisomerase-DNA conjugates and investigated their
processing by human cell-free extracts. A suicide DNA substrate was
constructed that upon reaction with vaccinia type I topoisomerase
yielded a highly stable covalent DNA-protein cross-link. When this
conjugate was treated with human nuclear or whole cell extracts, two
sites of DNA breakpoints were detected: one set of double-stranded
breaks occurred close to the 3' side of the topoisomerase (topo)
conjugation site, and there was another set of nicks about 30 nucleotides 3' to the topo site. The double-stranded breaks were not
made by extracts from xeroderma pigmentosum group A mutant cells,
suggesting that the xeroderma pigmentosum group A damage recognition
protein may be required for the occurrence of DNA breakage. In addition to these DNA breakage reactions, there was an activity that resulted in
the delinking of the frozen topoisomerase (or proteolytic fragments thereof) from the DNA substrate, which was followed by a ligation step
that restored the continuity of the broken DNA strand at the erstwhile
topo attachment site. We suggest that frozen topoisomerase-DNA conjugates (and perhaps other types of covalent DNA-protein complexes) are processed by multiple pathways that may involve the cleavage of the
DNA in the covalent protein-DNA complex and/or enzymatic delinking
followed by ligation of the broken DNA ends. These processes may
represent the "repair" of DNA-protein cross-links.
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INTRODUCTION |
Clear-cut evidence for the occurrence or the repair of DNA-protein
cross-links in normal living cells is lacking. Nevertheless, there are
compelling reasons to believe that the protein-DNA cross-link is a
distinct category in the repertoire of DNA damage in living cells (1).
Protein-DNA cross-links can occur via a variety of processes.
Extraneous agents such as UV (2) or therapeutic chemicals
(e.g. cisplatin, furocoumarins, and anticancer topoisomerase poisons) (3-6) are known to induce DNA-protein cross-links. Proteins contain reactive functional groups (NH2, COO
,
thiols, aromatics, and heterocyclics), which may react with the other
functional groups in DNA bases (7). In DNA, the 5,6 double bond of
pyrimidines is highly photoreactive (8). The cell is rich in
photosensitizers, such as porphyrins, cyanins, quinones, flavins, and
NADPH, which may act as UV-excitable photosensitizers that in turn may
react with protein functional groups via type I or type II mechanisms
(9, 10). Excited photosensitizers intercalated into DNA may react with
bound proteins to form ternary photoconjugates (4, 5, 11). The
intracellular milieu itself favors DNA-protein cross-linking.
Eucaryotic nuclear DNA is compacted >50,000-fold into chromatin, and
the DNA is always tightly associated with histones, high mobility group
proteins, transcription factors, polymerases, recombinases and
repair enzymes, which have typical macroscopic association constants in
the order of 1013-107
M
1. The stability time scales for protein-DNA
interactions range from several seconds to hours (12). Many DNA-binding
proteins that participate in recombination and replication form
transient covalent DNA-protein conjugates. Notable examples of this
category are topoisomerases, integrases, and replication initiator
proteins (13). Topoisomerase poisons can convert transient
topo1-DNA complexes into
highly stable frozen conjugates (6). Deleterious consequences can occur
if proteins covalently bound to DNA are not removed. There may be
arrest or premature termination of DNA replication and/or
transcription, which may result in chromosome instability and cell
death. Considering the above facts, we supposed that enzymatic
mechanisms capable of "repairing" protein-DNA cross-links should
exist in living cells. To discover such activities, we are designing
and developing covalently conjugated DNA-protein substrates. Here, we
have constructed a vaccinia topoisomerase I-DNA conjugate that
represents a well-defined paradigm for a DNA-protein cross-link. We
used this conjugate to test for activities in human cell-free extracts
that might be involved in putative repair pathways. We found that the
DNA in the cross-link was ruptured at specific sites by enzymatic
activities in the cell-free extracts. The frozen topoisomerase was
removed from its DNA attachment site, and the continuity of the DNA
strand appeared to be restored by a ligation step. We propose that
these reactions are steps in the pathways for the repair of frozen
DNA-topo conjugates, and perhaps for other types of DNA-protein
cross-links.
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MATERIALS AND METHODS |
DNAs and Proteins--
DNA oligos were purchased from
Midland Reagent Co. (Midland, TX). The concentration of DNAs were
determined from UV absorbance at 260 nm (
260
~104 M
1 cm
1/nt).
DNAs were either 5'-end labeled with T4 polynucleotide
kinase and [
-32P]ATP or 3'-end labeled with
[
-32P]dideoxy-ATP and calf terminal deoxynucleotidyl
transferase (14). The 32P-labeled and unlabeled DNAs were
run on preparative 8 M urea-polyacrylamide gels, and the
correct full-length DNAs were recovered by electroelution. Equimolar
amounts (1 µM) each of 58-mer, 94-mer, and 150-mer were mixed in kinase buffer (Tris-Mg buffer from New England Biolabs (Beverly, MA)), extracted with phenol-chloroform, and precipitated with
EtOH (14). The DNAs were taken up in 10 mM Tris-HCl (pH 7.5), 1 mM MgCl2/water, heated at 65 °C for
2-5 min, and cooled to room temperature over a period of 3-4 h.
Purified vaccinia topo I (fraction III9) was supplied by Drs.
JoAnn Sekiguchi and Stewart Shuman of the Memorial
Sloan-Kettering Cancer Center (New York, NY). The stock topo I
was at a concentration of 2.6 mg/ml in 50 mM Tris-HCl, pH
8.0, 1 mM EDTA, 2.5 mM DTT, 10% glycerol, 0.1% Triton X-100, 0.6-0.8 M NaCl. Twenty-five µl
aliquots of frozen topo I were stored at
70 °C. Two freeze-thaw
cycles did not affect its binding and covalent complex formation with
DNA.
Preparation of Cell-free Extracts and Cleavage Assay--
HeLaS2
nuclear extract was prepared and assayed for transcriptional activity
by Dr. Camilo Parada (in Dr. Robert Roeder's laboratory at Rockefeller
University) using the Dignam procedure (15). Nuclear extracts were at
~10 mg of protein/ml in 50 mM Tris-HCl, pH 7.9, 20%
glycerol, 0.2 mM EDTA, 5 mM DTT, 0.5 mM phenylmethylsulfonyl fluoride, 100 mM KCl.
HeLaS2 cells were obtained from Dr. Robert Roeder's laboratory, and
the human XPA mutant cell line (XP208, Japanese, complementation group
A-278700) was a gift of Dr. Peter Glazer (Yale University). Whole cell
extracts were prepared using the Manley method (16, 17) and stored in
25 mM HEPES-KOH pH 7.9, 100 mM KCl, 12 mM MgCl2, 0.5 mM EDTA, 2 mM DTT, 16% glycerol at 15 mg of protein/ml. Extracts were
frozen in 10-20-µl aliquots at
70 °C. Frozen extracts were
stable for several months, and the aliquots were not used after two
freeze-thaw cycles. All experiments were carried out with the same
batches of extracts. Our assay procedure was designed after a DNA
repair assay that was originally developed in Dr. Aziz Sancar's
laboratory (17). The details of the procedure were kindly supplied by
Drs. J. Reardon and Aziz Sancar. First, ice-cold master mixture
(usually 110 µl) was prepared. This consisted of 2 mM
ATP, 20 µM dNTPs, 200 µg/ml of bovine serum albumin,
100 ng of pUC plasmid DNA, buffer (30 mM HEPES-KOH, pH 7.9, 40 mM KCl, 3.2 mM MgCl2, 0.2 mM DTT, 0.1 mM EDTA), Tris salts (10 mM Tris-HCl, pH 7.5, 40 mM NaCl, 0.2 mM DTT, 0.2 mM EDTA), 32P-DNA
suicide substrate (final amount, 0.1 pmol; or with topo at 2-4-fold
molar excess), 20 µl of H2O per reaction (all are final
concentrations). The cell-free extracts were quickly thawed, and
different amounts were aliquotted into ice-cold Eppendorf tubes. The
master mixture (20 µl per reaction) was added to the extract, mixed
quickly, and kept at 30 °C for 1-3 h. Then, SDS (final
concentration, 1%), 22 µl of water, and proteinase K (final concentration, 20 mg/ml) were added, and the tubes were kept at 37 °C for 15 min. Finally, 150 µl of water was added, and the DNA
was extracted with a mixture of phenol:chloroform:isoamyl alcohol and
precipitated with EtOH (14). The DNA was redissolved in 12 µl of 8 M urea-TBE dyes and run on 15% acrylamide-8 M
urea-Tris borate-EDTA gels. The DNA bands were visualized by
autoradiography with x-ray films and/or a phosphor screen.
Native Gel Electrophoresis of DNA-Protein Complexes and the
Recovery of DNA--
The ds 150-mer suicide DNA substrates were
constructed with the 32P-label on the 5'-end of either the
58-mer or the 94-mer. The DNA substrates were run on a preparative
nondenaturing 8% acrylamide gel (12 cm long × 16 cm wide) in
Tris borate-EDTA buffer (14) and the radiolabeled 150-mer ds DNA bands
were identified by autoradiography of the wet gel. The appropriate
bands containing the DNAs were excised from the gel, and the DNA was
recovered by electroelution followed by extraction with
phenol:chloroform:isoamylalcohol and precipitated with EtOH (14).
Topoisomerase-DNA complexes were prepared by mixing vaccinia topo (0.6 pmol) with the purified ds 150-mer 32P-DNAs (0.3 pmol) in
15 µl of 50 mM Tris-HCl (pH 7.5). The reactions were
incubated at 37 °C for 10 min. Two to 5 µl of the topo-DNA complexes were then added to ice-cold Eppendorf tubes containing 2-5
µl of whole cell extract in a master mix (see above) or a compensating volume of only the master mix without the extract. The
final volume was 25 µl per reaction. The reactions were incubated at
30 °C for various lengths of time (see legend in Fig. 7). The reactions were then made either 5% glycerol or 5% glycerol plus 1%
SDS (without the tracking dyes) and were loaded in nondenaturing 5%
acrylamide gels (12 cm long × 16 cm wide) that were prepared and
run in Tris borate-EDTA buffer (14). The gels were electrophoresed at
60 V until the bromphenol dye (loaded in a gel well away from the
experimental samples) was 3-4 cm from the bottom of the gel. The gels
were then fixed in 5% acetic acid, 3% glycerol, 5% methanol for
30-40 min and dried. The DNA bands were visualized by autoradiography with an x-ray film.
In cases where the DNAs or the DNA-protein complexes were to be
recovered from native gels, the gel-fixing step was omitted and the wet
gels were directly autoradiographed. Gel pieces containing the DNA
bands of interest were excised from the gel and were soaked in 0.5 ml
of TE buffer (14), 0.3% SDS, 1 mg of proteinase K per ml for 2 h
at 37 °C. The gel pieces along with the solution were transferred in
to the cell of an electroelution (ELUTRAPTM) device (Schleicher & Schuell). The DNA was electroeluted and recovered after extraction with
phenol:chloroform:isoamylalcohol and precipitation with EtOH (14). Each
of the DNA samples was resuspended in 15 µl of TBE buffer containing
8 M urea, tracking dyes and heated in a boiling water bath
for 5 min and analyzed on thin 15% acrylamide-8 M urea-TBE
denaturing gels. The DNA bands were visualized by autoradiography.
 |
RESULTS |
The Topoisomerase-DNA Complex as an Example of Protein-DNA
Conjugates--
The monomeric (molecular mass, 36.6 kDa) type I
topoisomerase of the vaccinia virus specifically binds and cleaves the
conserved sequence 5'-(C/T)CCTT
in duplex DNA (18). During a single
catalytic cycle, the enzyme cleaves only the top strand (the scissile
strand) and forms a covalent enzyme-DNA intermediate (Fig.
1). Religation of the broken DNA bond
occurs following strand passage/rotation. The covalent intermediate has
a 3'-phosphodiester linkage with Tyr-274 (19, 20).

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Fig. 1.
The sequence of the suicide substrate
(A), diagrammatic representation of the suicide substrate
(B), and topo-DNA conjugate (C). The topo
binding site is shown in boldface letters, and the
arrow indicates the topo cleavage site.
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Our DNA substrate consisted of a 32P-58-mer, which ended in
CCCTTAT-3' (the cleaved strand) plus a cold 5' phosphorylated 94-mer top strand and an unlabeled 150-mer bottom strand (Fig. 1). In this DNA
substrate, there is only a single high-affinity consensus topo site
(CCCTT). Topo cleavage is highly specific at this site; even single
base substitutions in the consensus site reduce DNA cleavage quite
drastically (21). To demonstrate the formation of a stable covalent
frozen intermediate, we titrated a fixed amount of
32P-58-mer DNA "suicide" substrate with different
amounts of topo I. The reactions were then stopped by adding 1% SDS,
which denatured the topo. The samples were electrophoresed through 6%
acrylamide-SDS gels. In these assays, we used two variations of the
suicide substrate. In the first type of substrate, the
32P-58-mer cleavable strand was annealed to the unlabeled
150-mer bottom strand (i.e. the 94-mer was not included;
Fig. 1B without the 94-mer). This resulted in a partially
duplex structure with the topo cleavable site in the ds region. In the
second case, all the three oligos (32P-58-, 94-, and
150-mer; Fig. 1A), were annealed to create a fully ds
substrate shown in Fig. 1B. In the first substrate, topo I cleaves off the 3' overhanging terminal AT flap of the
32P-58-mer resulting in a frozen SDS-resistant
32P-56-mer-topo I conjugate (Fig.
2A). Because the 94-mer is
absent, ligation is prevented and the complex remains frozen. In the
second substrate, the frozen 32P-56-mer-topo I conjugate
remains so in the presence of SDS because the 5'-end of the 94-mer was
phosphorylated (Fig. 2B). The reason for testing both
substrates is that the substrate without the 94-mer served as an
"authentic control" suicide substrate because similar forms have
often been used in topo binding assays (see, for example, Ref. 22).
Thus, we can estimate the amount of frozen complex formed on the fully
ds substrate relative to the authentic control. With the partially ds
substrate a single complex representing the trapped topo-DNA conjugate
was seen (Fig. 2A, lanes 2-6). Based on the relative
phosphor counts in the free DNA versus cross-linked DNA, we
estimated that on the average about 85-90% of the 32P-DNA
was conjugated to the topo. A 2-4 fold molar excess of topo I was
sufficient to produce the maximum amount of frozen conjugate. Similarly, when the fully ds substrate, which contained the cold phosphorylated 94-mer was used, up to 90% of the
32P-56-mer DNA was conjugated to the topo (Fig.
2B). The presence of excess topo did not change the amount
of frozen complex formed. These experiments demonstrate that
SDS-resistant stable topo-DNA complexes were consistently formed on the
ds 150-mer suicide substrate. A small amount of free DNA was always
seen whether the phosphorylated 94-mer was present or not. This may be
simply because the conjugation reaction is never 100% efficient (just
like almost all in vitro enzymatic reactions), or it may be
because a small fraction of the DNA molecules were, for some reason,
not "liked" by the topo I. We do not believe that the probable
presence of a small fraction (10-15% or less) of free DNA in the
reactions would affect the interpretation of our subsequent
experimental results because a major fraction of the DNA is always
covalently bound to the topo I. Any free topo is unlikely to have
interacted with other sites on the DNA because we have not observed
multiple complexes in SDS gels and the cleavage by topo is highly
specific. Addition of Escherichia coli DNA polymerase Klenow
fragment (plus all four cold dNTPs) to the frozen topo-DNA conjugates
did not result in extension of the 5' 32P-labeled 56-mer,
indicating that the 3'-end of the 56-mer was inaccessible to Klenow
because it is conjugated to the topo (not shown).

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Fig. 2.
Covalent complexation of topo I with DNA
suicide substrate. Autoradiograms of 6% acrylamide-SDS gels.
Vaccinia topoisomerase I was purified from overexpressing E. coli cells (33). Various amounts of Topo I were mixed with
32P-labeled 58-mer DNA suicide substrate (1.3 pmol) in 17 µl of ice-cold 50 mM Tris-HCl (pH 7.5) and incubated at
37 °C for 10 min. The reaction was brought to 1% SDS and loaded on
6% acrylamide-SDS gels. The gels were dried and the bands were
visualized by autoradiography with an x-ray film. Panel A
was with DNA substrate without the 94-mer. Lane 1 is without
topo. Lanes 2-6 are with DNA plus topo at molar ratios of
1:1, 1:2; 1:4; 1:10, and 1: 20, respectively. Panel B: lanes
1-7 are with a fully ds substrate. Lane 1 is without
topo. Lanes 2-6 are with DNA plus topo at molar ratios of
1:1, 1:2; 1:4; 1:10, and 1: 20, respectively. Lanes 7 and
8 are DNA-topo (molar ratio at 1:2) complexes plus 5 µl of
whole cell extract using either a fully ds substrate (lane
7) or a partially ds substrate (lane 8). Panel C:
lane 1 contained ds DNA substrate without topo. Lanes 2 and 3 are with DNA and with extract plus topo (DNA:topo at a
molar ratio at 1:2) with either a partially ds (lane 2) or a
fully ds (lane 3) substrate, wherein the extract was first
added to DNA and then followed by the topo. Lane 4 is fully
ds DNA plus 5 µl of whole cell extract.
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We have performed additional control experiments to assess the binding
of topo I to a nonsuicide substrate (i.e. continuous top
strand plus bottom strand or with a nonphosphorylated 94-mer) relative
to the suicide substrate. In this nonsuicide substrate the topo I
recognition site was the same as that shown in Fig. 1. Using our gel
assay, we observed only a very faint band that represented an
equilibrium complex on the nonsuicide DNA substrate. The amount of
SDS-resistant topo-DNA complex that was formed on nonsuicide substrate
was only about 10% compared with that formed on the suicide
substrate.
When human cell-free extracts are added to topo-DNA conjugates,
super-shifted bands were seen (Fig. 2B, lanes 7 and
8). These SDS-resistant complexes may represent higher-order
complexes between topo-DNA conjugates and some factor(s) in the whole
cell extracts. In the absence of the topo there were no SDS-resistant
shifted bands with cell-free extracts alone (Fig. 2C, lane
4). Addition of extract, first, to the DNA and then the addition
of topo dramatically reduced complex formation, suggesting that
preformed topo-DNA complexes were required for the formation of
higher-order complexes (Fig. 2C, lanes 2 and 3).
These experiments demonstrated that human whole cell extracts contained
factors that tightly bind frozen topo I-DNA conjugates.
Breakage of DNA in the Protein-DNA Complex--
Initially, to
assay the fate of frozen topo-DNA cross-links, we attempted to work
with purified topo-DNA conjugates (i.e. without any free
DNA), which were obtained either by excising the DNA bands representing
the conjugates in native gels and subsequently electroeluting the
complexes, or by purifying topo-DNA reactions through a high
performance liquid chromatography gel-filtration column. With the
former procedure, we could recover only about 5-10% of the topo-DNA
conjugates even after prolonged (24 h) electroelution. Hence, this
procedure was abandoned because it was extremely inefficient. The high
performance liquid chromatography gel filtration procedure was somewhat
more efficient for separating free DNA from topo-DNA conjugates (data
not shown). However, with both procedures, the quality of the recovered
topo-DNA conjugates, as assessed from the UV absorption spectra, was
uncertain. The isolated topo-DNA conjugates did not show the expected
dominant broad transition at
max at 260 nm, which is a
signature of nondenatured native DNA-protein conjugates because the
stronger nucleic acid base transitions (centered at 260 nm) dominate
those from aromatic (e.g. Trp and Tyr) residues of the
protein. We concluded that the isolation and purification of native
topo-DNA conjugates was fraught with difficulties and that the isolated
conjugates may not represent native states. Hence, we decided to carry
out the following experiments with topo-DNA conjugates without further purification.
We incubated topo-DNA conjugates that were formed on the suicide
substrate with human nuclear or whole cell-free extracts. The extracts
were prepared by published procedures (see "Materials and
Methods"). The biochemical competence of these extracts was ascertained by performing a transcription assay driven by the adenovirus major late promoter (assay was carried out in Prof. Robert
Roeder's laboratory at Rockefeller University). When increasing amounts of human nuclear extract were incubated with preformed topo
I-DNA conjugates followed by digestion with proteinase K, a new band(s)
migrating at ~88 nt was seen (Fig.
3B, lanes 3-8, arrow). This
band suggested breakage of the topo-conjugated DNA substrate (see below
for more details). In the presence of topo I without the extract
(following treatment with proteinase K) or in the absence of topo I
with extract (following treatment with proteinase K), no such bands
were observed. This indicated that cleavage occurred only in the
presence of the extract and topo I (Fig. 3B, lanes 3-8).
These results were replicated several times. A full-length top strand
32P-150-mer (Fig. 3, arrowhead) was generated
whether topo I was present or not. The diamond symbol (Figs.
3 and 4) denotes slightly retarded bands
running just above the 56-mer (Fig. 3B, asterisk). These
bands may have originated from the 32P-56-mers that were
covalently linked to very short peptides that were proteinase
K-resistant.

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Fig. 3.
Treatment of topo I-DNA conjugates with
nuclear extract. Autoradiogram of a 15% acrylamide-urea gel. In
panels A and B, only the 58-mer in the substrate
was 32P-5'-end labeled (asterisk). Panel
A: lane 1, topo complexes were treated with proteinase K;
lane 2, DNA substrate was treated with proteinase K;
lanes 3-8, DNA substrate without topo treated with 0.5, 1, 2, 2.5, 3, and 5 µl of nuclear extract, respectively. Panel B:
lane 1, topo-DNA complexes were treated with proteinase K;
lane 2 contained DNA-topo complexes that were treated with
0.5 µl of a 10× dilution of extract; lanes 3-8 were
DNA-topo complexes that were treated with 0.5, 1, 2, 2.5, 3, and 5 µl
of nuclear extract, respectively. In panels C and
D, the DNA substrate was 5'-end labeled on the 150-mer
bottom strand. Panel C: lane 1, DNA treated with proteinase
K; lane 2, topo-DNA complexes were treated with proteinase
K; lanes 3-6, DNA substrate without topo that was treated
with 0.5, 2, 3, and 5 µl of nuclear extract, respectively.
Panel D: lane 1, DNA treated with proteinase K; lane
2, DNA-topo complexes were treated with proteinase K; lanes
3-6, DNA-topo complexes treated with 0.5, 2, 3, and 5 µl of
nuclear extract, respectively. The other lanes contained ss DNA
markers.
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Fig. 4.
Topo-DNA complexes were treated with whole
cell extracts. Note that only the 58-mer was 32P
labeled in this assay (asterisk). Panel A: lane 1 contained DNA-topo complexes without the extract that were treated with
proteinase K; lanes 2-8 contained DNA that was treated with
0.5 µl of a 10× dilution of extract and with 0.5, 1, 2, 3, 5, and 15 µl of extract, respectively. Panel B, lane 1 contained
topo-DNA complexes that were not treated with proteinase K; lanes
2-8 are DNA-topo complexes were treated with 0.5 µl of a 10×
dilution of extract and with 0.5, 1, 2, 3, 5, and 15 µl of whole cell
extract, respectively. The other lanes contained ss DNA markers.
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Fig. 4B (lanes 3-7) shows that whole cell
extracts generated the ~88-mer, just as the nuclear extracts did,
probably indicating that the cleavage activities resided in the
nucleus. We do not know why the highest amount of extract tested
inhibited the production of the approximately 32P-88-mer
(Fig. 4B, lane 8). It is possible that the cleavage activity aggregated and fell out of solution or there was an excess amount of
inhibitor(s) at the highest amount of extract; alternatively, the DNA
may have been sequestered by nonspecific DNA-binding proteins in the
extract.
Mapping the DNA Breakpoints--
To define the sites of the DNA
breakage, we 32P-labeled the ds suicide substrate on either
the 94-mer or the 150-mer (Fig. 1). Our attempts to assay for cleavage
of the 5'-end labeled 150-mer bottom strand were frustrated because the
nuclear and/or the whole cell extract contained a strong phosphatase
that for unknown reasons specifically dephosphorylated the
5'-32P-end of the bottom strand (Fig. 3, C and
D in Fig. 3). To circumvent this problem, we
32P-labeled the 3'-end of the 150-mer bottom strand using
[
-32P]dideoxy-ATP and terminal deoxynucleotidyl
transferase. As shown in Fig.
5A, the cleavage of the bottom
strand generated a cluster of bands that were predominantly centered at
~62 nt (Figs. 5A, 6II
150B-C, and 5D). This indicated that the bottom strand,
which is complementary to the topo-conjugated strand (i.e.
the scissile top strand), is also cleaved (Fig. 5D). (Note
that the topo I itself does not cleave the bottom strand.) When the
94-mer was 32P labeled, fragments measuring ~65 nt from
the 3'-end (Figs. 5B and 6, 94-C) or ~32 nt
from the 5'-end were seen (Fig. 5C). The ~65 nt and the
~32 nt fragments represented cleavage of the suicide substrate 3' to
the topo site (Fig. 5D). When cell-free extract was added to
a mixture of non-suicide DNA plus topo I, we did not observe specific
cuts in the DNA (not shown). This experiment indicated that the
presence of frozen topo-DNA conjugates was necessary for DNA
cleavage.

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Fig. 5.
Treatment of topo-DNA complexes with whole
cell extracts in which either the 150-mer bottom strand or the 94-mer
top strand were 32P-labeled. Panel A was with
the substrate labeled at the 3'-end of the 150-mer bottom strand.
Lane 1 contained only ds suicide substrate. Lanes
2-6 are topo-DNA complexes that were treated with different
amounts (1, 2, 3, 4, and 5 µl, respectively) of whole cell extracts.
Lane 7 was with 5 µl of whole cell extract alone, whereas
lane 8 was with topo alone. Panel B, the
substrate contained a 94-mer top strand that was
32P-labeled at its 3'-end. Lanes 1-5 are
topo-DNA complexes that were treated with different amounts (1, 2, 3, 4, and 5 µl, respectively) of whole cell extracts. Lane 6 was with 5 µl of whole cell extract alone, and lane 7 contained only the substrate with topo. Panel C, the
substrate contained a 94-mer top strand that was 32P
labeled at its 5'-end. Lane 1 contained DNA with topo alone.
Lane 2 contained topo-DNA complexes that were treated with 5 µl of whole cell extract. Lane 3 was with 5 µl of whole
cell extract alone. All reactions were finally treated with proteinase
K. Panel D, schematic illustration of approximate positions
of the cleavage sites. The 5' and 3' 32P-labeled ends are
denoted by the asterisk.
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Fig. 6.
Isolation and sequencing of the breakage
products. Panel I, the 32P label was on the 5'
of the 58-mer. Lane 1, G-reaction of the 150-mer top strand
that was isolated from gels such as that shown in Fig. 4B.
Lane 2, G-reaction of the 88-mer that was isolated from gels
such as that shown in Fig. 4B. Lanes 3 and
4 contained synthetic DNA markers. Panel II,
isolated and piperidine-reacted 94-mer-derived (94-C)
fragments. The 32P label was on the 3'-end of the 94-mer.
Three different amounts of DNA (~10-30 fmol) were loaded on the gel.
T and B denote top and bottom fragments,
respectively. Isolated and piperidine-reacted 150 bottom strand-derived
products (150 B-C) were from gels such as that shown in Fig.
5A. Three different amounts (~2-30 fmol) from three
separate cleavage reactions were loaded on the gel. Std
indicates ss DNA markers. Panel III is a G-ladder that was
derived from the gel-purified cleavage products from topo-DNA 3'-end
labeled 150-mer bottom strand. Lane 1 contained DNA that was
not reacted with dimethyl sulfate but was reacted with piperidine.
Lane 2 contained DNA that was reacted with dimethyl sulfate
and cleaved with piperidine. Panel IV is a G-ladder derived
from cleaved fragments from a substrate that was the 3'-end labeled on
the 94-mer. Lane 1 contained DNA that was not reacted with
dimethyl sulfate but was reacted with piperidine. Lane 2 contained DNA that was reacted with dimethyl sulfate and cleaved with
piperidine. The numbers on the right side of each panel
refer to the position of the Gs (darker bands) and As
(fainter bands) from their respective 32P ends.
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To map the fragmentation pattern in a better way and to rule out the
possibility that very short peptides (end-products of proteinase K
treatment) may be still attached to the DNAs (thus complicating the
assignment of the cleavage sites), we isolated the cleaved products
from the denaturing gels, such as those in Figs. 4 and 5. The isolated
cleaved DNA fragments were extracted with phenol:chloroform,
precipitated with EtOH, and then subjected to Maxam-Gilbert G-reaction
with dimethyl sulfate followed by hot piperidine or with hot piperidine
alone (23). We reasoned that this harsh procedure (hot alkali, etc.,)
will hydrolyze any peptide-DNA bonds but will leave most unmodified
naked DNA molecules intact. Fig. 6 shows a collection of isolated
fragments after such a treatment. Surprisingly, the
32P-88-mer (shown by the arrow in Fig. 4), after
gel isolation and Maxam-Gilbert G-reaction, migrated at 62 nt in a high
resolution denaturing gel (Fig. 6I, lane 2). There is indeed
a faint ~88-mer band in Fig. 6I, lane 2 representing the
residual nonhydrolyzed DNA-peptide conjugate. There were no other bands
between the faint residual ~88-mer band and the much heavier 62-mer
band (Fig. 6I, lane 2), indicating a clean quantitative
hydrolysis. These data indicated that some proteinase K-resistant
peptides were indeed attached to this DNA fragment, resulting in its
anomalous migration at ~88 nt in the previous gel (Fig. 4) and that
these DNA-linked peptide fragments were cleaved off during the
subsequent purification and/or the harsh alkali treatment (see above).
The sequence of this 62-nt fragment perfectly matched the sequence of
the full-length 150-mer top strand (which was purified from gels such
as that represented by the triangle in Fig. 4) starting with
the 62nd-nt breakpoint and counting from the 5'-end (compare Fig.
6I, lanes 1 and 2). The alternative explanation
that peptide fragments from topo that were still covalently conjugated
to the 32P-56-mer resulted in its anomalous migration to
62-mer (in Fig. 6I, lane 2) is unlikely because cleavage of
the tyr-phosphodiester bond in hot alkali will release a 56-mer but not
a 62-mer (see Ref. 22 for a discussion of the stability of the
tyr-phosphodiester). These results indicated that at least in a
fraction of the topo-DNA conjugated suicide substrate molecules, there
was a cleavage of the DNA 3' to the topo attachment site. Because such
a cleavage resulted in ~62 nt with sequences from the 94-mer, there
might have been a ligation event with the 94-mer (see below for more details). Our extracts did posses ligase activity (and the reactions contained added ATP) because the top strand 32P-150-mer was
also produced in the absence of the topo (Fig. 3, A and
B and Fig. 6I, lane 1). To find out whether
ligase itself was sufficient to generate the 150-mer top strand in the
topo-DNA complexes, we incubated topo-DNA conjugates that were formed
on the suicide DNA substrate with T4 DNA ligase alone (plus ATP) and/or
supplemented with extracts (not shown). In the presence of T4 DNA
ligase alone, we did not observe the formation of the 150-mer top
strand. However, when the reaction was supplemented with whole cell
extracts, we observed the 150-mer and DNA breakage. This suggested that
ligase by itself was insufficient for the formation of the 150-mer top
strand from the topo-DNA conjugates and that, at least in some topo-DNA
conjugates, the extract was needed to first remove the topo I before
ligation occurred (see below for more evidence).
The 94-mer and 150-mer-derived cleaved fragments (Figs. 5,
A-C and 6II) appeared as multiple bands
indicating a clustered cleavage pattern (Fig. 5D). The
cleavages at ~32 nt from the 5'-end of the 94-mer (downstream to the
topo-conjugated site; Fig. 5, C and D) appear to
be single-stranded nicks because we did not observe breakage at the
corresponding place on the 150-mer bottom strand. We isolated and
performed G-reactions on the cleaved 3'-end labeled 94-mer and the
bottom 150-mer-derived fragments (Fig. 6, III and
IV). The sequence and the positions of the breakpoints are
consistent with the fact that cleaved fragments originated from 3'-end
labeled top strand 94-mer and the bottom strand 150-mer (Fig. 6,
III and IV).
The composite DNA breakpoints are shown in Fig. 5D. Note
that the approximate sizes of the fragments are given, consistent with
the clustered appearance of the cleaved bands and the decreased resolution toward the top of the gels (± 3-5 nt). In summary, it
appears that the major breakage in the topo-DNA conjugate was a cluster
of ds breakpoints immediately 3' to the topo-conjugation site. In
addition, ss nicks downstream of the ds breakpoints were seen on the 3'
side of the topo-conjugated site. These breakpoints are not due to
footprinting by unbound topo of some other proteins from the extracts
that may be bound to the DNA substrate because the topo is very
specific to the unique consensus sequence at the end of the 56-mer topo
strand.
A DNA Ligation Event Is Carried Out by the Extracts--
Earlier,
we suggested that the 150-mer top strand was formed by ligation of the
56-mer to the 94-mer in the presence of the extract and topo (Figs.
3B and 4B). To prove that this is indeed the
case, we examined isolated complexes from native gels (Fig. 7). In a reaction containing topo-DNA
conjugates plus extract, without the addition of SDS, the complexes did
not migrate very far into the native gel (Fig. 7, I and
II). When SDS was added to the reaction mixture before
deposition into the gel wells, the complexes migrated into the gel
(Fig. 7, III and IV). Prolonged incubation of the
topo-DNA complexes with extract resulted in appearance of smeary bands
(which were retarded compared with the free 150-base pair substrate)
representing various species of DNA-protein complexes (Fig. 7IV,
lanes 1-4). To test the possibility that these protein-bound DNA
molecules could contain ligation products, we isolated the DNA from the
complexes (such as those indicated by brackets A-C in Fig.
7IV, lanes 1-4) after in situ proteinase K
digestion (see "Materials and Methods"). (Without the proteinase K
treatment, the DNA recoveries were extremely poor.) The isolated DNA
was subsequently run on 15% acrylamide-denaturing gels. Fig. 7,
V and VI, shows that the 150-mer top strand is
indeed formed in complexes whether the DNA substrate was
32P-labeled on the 5'-end of the 58-mer or the 94-mer. That
the 150-mer top strand was generated by ligation and not by nick
translation is apparent because: 1) the 150-mer is generated from the
substrate containing 32P only on the 94-mer. If nick
translation by a DNA polymerase were the mechanism, there would not be
a 32P label on the 150-mer because DNA polymerase cannot
extend a 5'-phosphoryl end. 2) No intermediate-sized products between
56 (or 94) and 150-mers were seen when the 32P label was on
the 58-mer or the 94-mer. Nick translation reactions are generally not
100% processive for all DNA polymerase molecules that initiate. Hence,
normally paused or prematurely terminated products are usually seen
with in vitro nick translations. However, in Fig. 7,
V and VI, lane 1, no intermediate-sized DNA
molecules were seen, even in overexposed autoradiograms. There was no
appreciable phosphatase activity because the 5' 32P label
persisted. These results indicate that ligation probably joined the
56-mer to the 94-mer in some complexes in the mixture of topo-DNA-cell
extract, presumably after the delinking and dephosphorylation of the
3'-topo linkage. Consistent with previous results (Fig. 4) some amount
of the 150-mer top strand was formed in the absence of topo with the
extract (Fig. 7, VI and V, lane 2).

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Fig. 7.
Panels I-IV, native gel electrophoresis
of topo-DNA-extract complexes. The figure shows autoradiograms of 5%
acrylamide nondenaturing gels that were made and run with TBE buffer.
Panel I, the suicide DNA substrate was 32P
labeled at the 5'-end of the 58-mer. Lane 1, suicide DNA
alone; lane 2, suicide DNA + topo; lane 3,
suicide DNA + topo + 3 µl of whole cell extract; lane 4,
suicide DNA + 3 µl of whole cell extract. All reactions were
incubated at 30 °C for 2 h. Panel II, the suicide
DNA substrate was 32P labeled at the 5'-end of the 94-mer.
Lane 1, suicide DNA alone; lane 2, suicide DNA + topo; lane 3, suicide DNA + topo + 3 µl of whole cell
extract; lane 4, suicide DNA + 3 µl of whole cell extract.
All reactions were incubated at 30 °C for 2 h. In panels
I and II, the samples did not receive SDS after the
incubation time. Panel III, the suicide DNA substrate was
32P labeled at the 5'-end of the 94-mer. Lane 1,
suicide DNA alone; lane 2, suicide DNA + topo; lane
3, suicide DNA + topo + 3 µl of whole cell extract; lane
4, suicide DNA + 3 µl of whole cell extract. All reactions were
incubated at 30 °C for 15 min. Panel IV: lane 1,
suicide DNA 32P labeled at the 5'-end of the 58-mer + topo + 3 µl of whole cell extract were incubated for 2 h; lane
2, suicide DNA 32P labeled at the 5'-end of the 94-mer + topo + 3 µl of whole cell extract were incubated for 4 h;
lane 3, suicide DNA 32P labeled at the 5'-end of
the 94-mer + topo + 3 µl of whole cell extract were incubated for
2 h; lane 4, suicide DNA 32P labeled at the
5'-end of the 94-mer + 3 µl of whole cell extract were incubated for
2 h. All incubations were done at 30 °C. Brackets A-C
indicate examples of regions in the gels that were excised to extract
the DNAs after in situ proteinase digestion (see
"Materials and Methods"). Panels V and VI,
autoradiograms of 15% acrylamide-8 M urea gel showing the
DNA that was isolated from complexes that were previously run on native
gels (see "Materials and Methods"). Panel V: lanes 1-3
contained suicide DNA substrate that was 32P labeled on the
5'-end of the 58-mer. Lane 1, DNA that was extracted from
complexes from reactions containing topo + whole cell extract (as in
A, panel IV); lane 2, DNA that was extracted from
complexes from the reactions containing whole cell extract; lane
3, DNA that was extracted from complexes in the reactions
containing topo only. Panel VI: lanes 1-3 contained suicide
DNA substrate that was 32P labeled on the 5'-end of the
94-mer. Lane 1, DNA that was extracted from complexes in the
reactions containing topo + whole cell extract (as in B, panel
IV); lane 2, DNA that was extracted from complexes in
the reactions containing whole cell extract (as in C, panel
IV); lane 3, DNA that was extracted from complexes in
the reactions containing topo only.
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In discussing Fig. 5D, we implied that ds cuts occurred in
the vicinity of the topo-conjugated site. Double-stranded fragments equivalent to 60-65 base pairs representing these cuts (migrating, as
expected, with bromphenol blue dye) were seen in native gels (Fig.
7IV, lanes 1-3). Fragments from reactions involving topo or
with the cell extract alone were of a different length. These results
indicated that 60-65-base pair ds cuts are made in reactions containing DNA-topo conjugates treated with cell extracts. The larger
DNA molecules running just above the 60-65-base pair fragments (Fig.
7IV, lanes 1-3) may represent fragments with bound proteins and/or other cleaved fragments.
XPA Protein Is Required for the Major DNA Breakage 3' to the topo
Site--
The XPA mutant cells are drastically reduced in the repair
of UV-induced bulky adducts (24). The XPA protein is essential for the
nucleotide excision repair pathway (and perhaps for other repair
pathways as well) (24). XPA is a damage recognition protein, and
interacts with several other proteins in the nucleotide excision repair
pathway (24-26). We wanted to test whether XPA protein played a role
in DNA cleavage activities reported here. Fig.
8B shows that XPA mutant whole
cell extract cannot produce the 32P-88-mer band (which is
actually 62 nt, as illustrated by isolation and sequencing; Fig.
6I, lane 2). Mixing XPA whole cell extract with wild type
whole cell extract restored the 32P-88-mer band (Fig.
8C, lane 2, arrow), indicating that at least for the top
strand breakage step near the topo, the XPA damage recognition protein
may be required. However, the top strand 150-mer (Fig. 8,
triangle) is produced with XPA extracts because, as shown above, it was most probably generated by a ligation step.

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Fig. 8.
Topo-DNA complexes were treated with whole
cell XPA extracts. Note that only the 58-mer was 32P
labeled in this assay (asterisk). Panel A: lane 1 is DNA that was treated with proteinase K; lanes 2-6 are
DNA substrates without topo that were treated with 1, 2, 3, 5, and 10 µl of extract, respectively. Panel B: lane 1 is DNA-topo
complexes that were treated with proteinase K; lanes 2-6
are DNA-topo reactions that were treated with 1, 2, 3, 5, and 10 µl
of extract, respectively. The other lanes contained ss DNA
markers. Panel C: lane 1, contained DNA-topo complexes that
were treated with 1 µl of XPA whole cell extract, whereas lane
2 contained 0.5 µl of XPA whole cell extract that was mixed with
0.5 µl of wild type whole cell extract.
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DISCUSSION |
This work provides evidence for mechanisms that process topo-DNA
cross-links. Some of the results may have relevance to the processing
of other types of protein-DNA cross-links as well. We report that the
DNA in a frozen topo-DNA conjugate was broken at specific sites by
nuclease activity in human cell extracts, and there appears to be a
mechanism that delinks the topo-DNA cross-link, perhaps by first
removing the frozen topo (or proteolyzed parts thereof) and then
restoring the continuity of the broken DNA strand by ligation. Our
observations are admittedly at the phenomenological level
("proof-of-principle"), and for that reason, we cannot yet propose
a repair pathway. Nonetheless, because the ds breakpoints are located
immediately 3' to topo-DNA conjugation site, there is an intriguing
possibility that these ds breakpoints may represent steps in the
processing of these conjugates. The dependence of the DNA breakage on
XPA, which is a damage recognition protein (26), suggests that a frozen
topo-DNA conjugate (or parts thereof) may be perceived by the cell as a
potential DNA damage problem. We cannot yet mechanistically or
temporally relate the ds break points and the topo delinking-ligation
activity. The production of the 150-mer top strand occurred via a
pathway that may first involve the delinking of the DNA conjugated
topo. While the early versions of this work were being written-up for publication, a report appeared describing a DNA phosphodiesterase that
is specific for phospho-tyrosyl bonds (27). These workers suggested
that the phosphodiesterase would remove a frozen DNA-topo. In our case,
this event may be followed by a phosphatase that converted the
erstwhile 3'-end to an OH, which is then ligated to the phosphorylated
94-mer. The temporal relationship, if any, between the ligation event
that generated the 150-mer and the cleavages 3' to the topo is unknown.
The breakage of the DNA in the frozen DNA-topo conjugate may be an
alternate pathway for the processing of frozen topo conjugates, in
addition to the topo-delinking-ligation pathway. It is possible that
the ds breaks may signify a breakage and rejoining step that may be
akin to recombinational repair of ds breaks and interstrand DNA
cross-links (see Ref. 26). Thus, frozen topo-DNA conjugates or other
types of cross-links may be processed by more than one route.
We chose the topo-DNA complex as a model substrate for our assay
because it is a very well-characterized example of a protein-DNA cross-link and because the chemical nature of the covalent linkage is
well established (see "Introduction"). Moreover, it is a naturally occurring cross-link. Frozen topo-DNA conjugates may be formed in
patients undergoing chemotherapy with anticancer topoisomerase poisons,
such as camptothecins (6). The topo I-DNA covalent bond is a
3'-phosphodiester linkage. Topo II-DNA complexes are 5'
phosphoryl-linked (6). UV and ionizing radiations also generate protein-DNA cross-links (26). Ether bonds or C---C bonds involving Tyr
and the ring carbons of thymine or cytosine are induced by far and near
UV or ionizing radiation or by photosensitizers, such as psoralen plus
near UV (2, 28-30). Other covalent linkages, such as Schiff's
bases, may also occur. If protein-DNA cross-links occur frequently, as
we believe they do, enzymatic pathways for their removal must be
ubiquitous. Accepting the principle of cellular parsimony, it is hard
to imagine that there is a distinct enzyme that can catalyze the
reversal of each type of covalent bond (akin to the
tyrosylphosphodiesterase for topo I (27)) in the wide variety of
protein-DNA cross-links. Therefore, we believe that the breakage of DNA
that we have demonstrated here may suggest a pathway that is capable of
handling a wide spectrum of protein-DNA cross-links. In this context,
it is worth noting that nucleotide excision repair can repair a wide
spectrum of DNA bulky adducts (24, 26). Some enzymes from the
nucleotide excision repair, base excision or mismatch, or
recombinational repair pathways may also participate in DNA-protein
cross-link repair.
Lastly, previous reports regarding protein-DNA cross-link repair are
difficult to relate to our work because those assays were carried out
with undefined substrates (31, 32). In conclusion, this work represents
experimental evidence toward an understanding of the mechanisms that
process topo-DNA cross-links and perhaps other DNA-protein
cross-links.
We thank the following colleagues: Drs. JoAnn
Sekiguchi and Stewart Shuman for the kind gift of vaccinia topo I,
advice, suggestions, and critical comments on an earlier version of
this paper; Drs. Joyce Reardon and Aziz Sancar for advice and
protocols; Dr. Camilo Parada for the generous gifts of HeLaS2 cells,
nuclear extracts, and the testing the transcriptional competence of
extracts; Dr. Peter Glazer for the gift of xeroderma pigmentosum group
A cell line; Dr. Vijayasarathi Setaluri for help with the cell culture and maintenance of cell lines; Dr. Paul Modrich for suggestions; and
Prof. Joshua Lederberg for his interest in the project.