(Received for publication, June 7, 1995; and in revised form, September 20, 1995 )
From the
Human chromatin-associated protein extracts were examined for endonucleolytic activity on a defined 132-base pair DNA substrate containing a single, site-specific 4,5`,8-trimethylpsoralen plus long wavelength ultraviolet light-induced furan side or pyrone side monoadduct or interstrand cross-link. These extracts produced incisions on both the 3` and 5` sides of each of these lesions. The distance between the 3` and 5` incisions at sites of a furan side monoadduct or cross-link was 9 nucleotides, and at sites of a pyrone side monoadduct or cross-link it was 17 nucleotides. Incisions on the 3` side of both types of furan side and pyrone side adducts were similar and were either at the fourth or fifth phosphodiester bond from the adducted thymine, depending upon the adduct. However, greater differences were observed between sites of 5` incision. This incision occurred at the fifth and sixth phosphodiester bonds from the adducted thymine at sites of furan side monoadducts and cross-links, respectively, and at the 13th and 14th phosphodiester bonds at sites of pyrone side monoadducts and cross-links, respectively. Thus, direct analysis of sites of endonucleolytic incision reveals that the location of sites of incision on TMP-adducted substrates depends upon the type of adduct present.
Repair of DNA interstrand cross-links is critical for a number
of cellular processes such as transcription and DNA replication and
therefore is particularly important for the maintenance of genetic
integrity and cellular survival. A nucleotide excision repair mechanism
is responsible in both prokaryotes and eukaryotes for the removal of
this type of lesion(1, 2, 3) . Although the
molecular basis of repair of DNA interstrand cross-links has been
extensively studied in bacteria, the proteins involved in the removal
of these lesions in mammalian cells and their mechanism of action is
largely unknown. A critical, rate-limiting step in this repair process
is the initial damage recognition and incision step in which a protein
specifically locates or recognizes a site of damage and an endonuclease
makes an incision on the DNA strand at or near this site. We have
recently identified a damage recognition protein in normal human
chromatin, which binds to DNA containing interstrand cross-links
produced by 4,5`,8-trimethylpsoralen (TMP) ()plus long
wavelength ultraviolet (UVA) light, and our studies suggest that it
plays a role in the repair of interstrand cross-links(4) .
However, an endonuclease that specifically incises DNA at sites of
these interstrand cross-links has heretofore not been isolated from
mammalian cells.
A number of different agents have been shown to produce interstrand cross-links in DNA. One of the most definitive of these, whose reaction with DNA has been well characterized, is psoralen plus UVA light. Psoralens are a group of three-ring heterocyclic furocoumarins that contain two reactive double bonds, a 4`,5`-double bond in the furan ring and a 3,4-double bond in the pyrone ring(5, 6, 7, 8) . Psoralen-DNA adducts are formed in three stages; psoralen first intercalates into the DNA duplex in a noncovalent manner and then, upon photoreaction with UVA light, forms either a furan side or a pyrone side monoadduct with the 5,6-double bond of a pyrimidine, which the majority of the time is a thymine. If the furan side of the molecule is linked to DNA, then further exposure to UVA light leads to production of an interstrand cross-link(5, 6, 7, 8, 9, 10) .
In Escherichia coli, the UvrABC nuclease is responsible for repair of psoralen-photoinduced monoadducts and interstrand cross-links. The UvrA, UvrB, and UvrC proteins act in concert to excise these lesions by making sequential incisions on both the 3` and 5` sides of the modified nucleotide(11, 12, 13, 14, 15, 16, 17) . Although repair of psoralen monoadducts and interstrand cross-links has been shown to occur in mammalian cells(18, 19, 20, 21, 22, 23, 24, 25, 26) , the precise nature of the initial events and proteins involved in removal of these lesions are less clear. We have isolated from the nuclei of normal human cells a chromatin-associated DNA endonuclease complex, pI 4.6, which recognizes and incises DNA containing TMP or 8-methoxypsoralen plus UVA interstrand cross-links and another, pI 7.6, which recognizes and incises DNA containing psoralen monoadducts(22, 23, 25, 27) . Each complex contains proteins involved in damage recognition, chromatin interaction, and endonucleolytic incision, and each has been demonstrated to be involved in the repair process by its ability to correct the repair defect in repair-deficient cells when introduced into them via electroporation (28, 29) .
In the present study, we describe the construction of a 132-base pair oligonucleotide substrate that contains a centrally placed single, site-specific TMP monoadduct or interstrand cross-link. These substrates are so constructed that endonucleolytic activity on either the furan side or the pyrone side of each type of adduct may be selectively studied. We have used these four substrates to directly examine the pattern of incision produced by human chromatin-associated protein extracts that contain both the endonuclease complexes, pI 4.6 and pI 7.6. Our results show that these extracts make incisions in DNA on both the 3` and 5` sides of each of the two types of furan side and pyrone side adducts (i.e. monoadducts and cross-links). The incision patterns on each of these four substrates are different. The distance between the 3` and 5` incisions made at sites of furan side monoadducts and cross-links is 9 nucleotides, whereas the distance between the 3` and 5` incisions made at sites of pyrone side monoadducts and cross-links is 17 nucleotides.
Cell nuclei were isolated, and the
chromatin-associated proteins were separated from the nucleoplasmic
proteins in a series of steps as described
previously(22, 31) . Chromatin-associated proteins
were dialyzed into 50 mM potassium phosphate (pH 7.1), 1
mM -mercaptoethanol, 1 mM EDTA, 0.25 mM
phenylmethylsulfonyl fluoride, and 40% ethylene glycol and then passed
through a CM Sephadex column and stored at -20
°C(4, 22, 31) . Protein concentrations
were determined by the Bio-Rad protein assay (Bio-Rad).
Figure 1: DNA substrate used in this study. A 132-bp DNA fragment was constructed with a 4-bp overhang on the 5`-end of each strand. Sites of ligation of the component oligonucleotides (ML1-ML7) are indicated by arrows. The numbering system between the top (ML8) and bottom (ML9) strands refers to the region of the 5 S rRNA gene from L. variegatus that was utilized.
Figure 2: Schematic for preparation of the 132-bp DNA substrates containing either a single TMP furan side monoadduct or cross-link or pyrone side monoadduct or cross-link. TMP is indicated by the angled lines. F and P refer to the furan and pyrone rings in TMP.
For construction of the furan side monoadduct, ML2 (180 pmol) was
dephosphorylated with calf intestinal alkaline phosphatase and
5`-end-labeled with 1 mCi of [-32P] ATP (6000
Ci/mM; DuPont NEN) by using T4 polynucleotide kinase
(Pharmacia). ML2 was freed from protein, salts, and unincorporated
label by passing through Nensorb 20 nucleic acid purification
cartridges according to the manufacturer's directions (DuPont
NEN). ML2 was then eluted with 50% ethanol and dried in a Speed Vac in
a siliconized eppendorf tube. To the dried pellet, 180 pmol of
5`-phosphorylated ML1, ML4, ML5, ML6, and ML7 and 540 pmol of ML3
oligonucleotide containing the furan side TMP monoadduct were added in
a total volume of 150 µl containing 10 mM Tris-HCl, pH
7.6, 0.4 mM EDTA, and 25 mM NaCl (Fig. 2). The
amount of the ML3 furan side monoadduct intermediate added was three
times that of the other oligomer components, since the efficiency of
ligation was dependent upon the amount of ML3 monoadduct in the
reaction mixture. The mixture was heated at 95 °C for 3 min, and
then the DNA strands were hybridized at 60 °C for 30 min followed
by slow cooling to 25 °C. To this mixture 100 units of DNA ligase
(Boehringer Mannheim) was added, and ligation was carried out at 12
°C for 90 min, followed by 4 °C overnight. Ligation was found
to be equally efficient when carried out at 22 °C for 1 or 2 h. The
ligated full-length product was isolated by electrophoresis of the
sample through a denaturing 5% polyacrylamide gel. As mentioned above
this product also contained the complementary unlabeled full-length
bottom strand (ML9). The purity of the duplex DNA was ensured by
repurifying this product on a nondenaturing 5% polyacrylamide gel (data
not shown). This product represented the furan side monoadducted
substrate in which TMP was adducted to a thymine at position 132 on ML8 (Fig. 1). The preparation of undamaged DNA substrate was similar
to this except that an undamaged ML3 sequence was used for this
purpose.
For construction of the furan side cross-linked DNA, half
of the above furan side monoadducted substrate in 100 µl of 10
mM Tris-HCl, pH 7.6, and 0.4 mM EDTA was irradiated
with the UVA light (20 milliwatts/cm) for 25 min at 20
°C (Fig. 2). The furan side cross-linked substrate was
purified by electrophoresis on a denaturing 5% polyacrylamide gel.
The strategy for formation of the pyrone side interstrand cross-link was similar to that used for the furan side cross-link. However, in this instance in the ligation reactions the ML6 rather than the ML2 oligonucleotide was 5`-end labeled (Fig. 2). This resulted in the bottom strand (ML9) being internally labeled rather than the top strand (ML8) (Fig. 1). In this substrate the pyrone ring of TMP was adducted to a thymine at position 133 on ML9 (Fig. 1A).
The pyrone side monoadducted substrate was prepared according to a protocol described by Yeung, Dinehart, and Jones(34) . The psoralen cross-link was reversed by treatment with hot alkali, which has been shown to specifically cleave the psoralen cross-link at the furan side, whereas the pyrone side monoadduct is relatively resistant to this treatment(34) . Half of the above pyrone side cross-linked substrate in 150 µl of 10 mM Tris-HCl, pH 7.6, 0.4 mM EDTA, and 100 mM KOH was heated at 90 °C for 30 min, and the product was purified on a denaturing 5% polyacrylamide gel (Fig. 2). The preparation of the undamaged DNA substrate was similar to this except that it contained an undamaged ML3 sequence on the top strand.
All of the above substrates were cleaned up with the Nensorb 20 nucleic acid purification cartridges according to the manufacturer's protocol (DuPont NEN) and were stored in aliquots at -20 °C in 10 mM Tris-HCl (pH 7.6) and 0.4 mM EDTA.
To construct the 132-bp furan side monoadducted DNA, TMP-modified ML3 was ligated together with the other six component oligomers. The upper strand was internally labeled on the 5`-end of oligonucleotide ML2 ( Fig. 1and Fig. 2). The ligation reaction occurred with over 50% efficiency. The purity of the duplex DNA was ensured by repurifying this product on a nondenaturing gel (data not shown). To form the furan side cross-linked DNA, the above monoadducted substrate was photoreacted with 365-nm UV light (Fig. 2). The formation of the furan side cross-link was also very efficient. Greater than 90% of the monoadducted substrate was converted to the cross-linked form (Fig. 3A, upper band, lane 1).
Figure 3:
Purification of the 132-bp furan side and
pyrone side cross-linked substrates and the 132 bp pyrone side
monoadducted substrate. A, lane 1, the furan side
monoadducted 132-bp substrate, internally labeled on the top strand,
was irradiated with UVA light (20 milliwatts/cm) to form a
furan side cross-linked substrate, which was purified in a denaturing
5% polyacrylamide gel (top band). Lane 2, the pyrone
side monoadducted 132-bp intermediate, internally labeled on the bottom
strand was irradiated with UVA light, as above, to form the pyrone side
cross-linked substrate, which was purified by gel electrophoresis (top band). B, the pyrone side cross-linked substrate
in panel A, lane 2, was reversed by treatment with
100 mM KOH at 90 °C for 30 min, and the resulting pyrone
side monoadducted substrate was purified on a denaturing 5%
polyacrylamide gel (bottom bands).
For construction of the pyrone side adducted substrates, TMP-monoadducted ML3, was used in the ligation reactions in which the bottom strand was internally labeled on the 5`-end of ML6 (Fig. 2). As was the case for the top strand, the ligation reaction for the bottom strand occurred with over 50% efficiency. The purity of the duplex DNA was ensured by repurifying this product on a nondenaturing gel (data not shown). The monoadducted 132-bp substrate was then driven to the cross-linked form by exposure to 365-nM UV light (Fig. 2). Greater than 90% of the monoadducted substrate was converted to the cross-linked from (Fig. 3A, upper band, lane 2). The lower bands in Fig. 3A, lanes 1 and 2, may represent unreacted full-length monoadducted substrates. To form the pyrone side monoadduct, the pyrone side cross-link was reversed by treatment with hot alkali (Fig. 2). The alkali reversal was also very efficient since the cross-linked substrate was completely reversed to the monoadducted form (Fig. 3B).
Figure 4:
Autoradiogram showing the normal human
endonucleolytic incision pattern of the 132-bp furan side monoadducted
or cross-linked substrate. Upper panel, the top strand, ML8,
was internally labeled with P. The thymine modified with
TMP is indicated by an asterisk. Maxam-Gilbert sequence
reactions G, G + A, T + C, and C are shown. Normal human
chromatin-associated protein extracts (10 µg) were incubated with
the TMP-adducted substrates, and the reaction products were analyzed on
a denaturing 6% polyacrylamide gel. The bands indicating incisions 3`
and 5` to the adducted thymine are denoted by arrows. Lane
1, pattern of endonucleolytic incision on the furan side
monoadducted substrate; lane 2, incision pattern on the furan
side cross-linked substrate; lane 3, undamaged substrate
treated with protein extract. Lower panel, sites of 3` and 5`
endonucleolytic incision on the furan side monoadducted substrate (A) and the furan side cross-linked substrate (B)
(indicated by arrows). The adducted Ts are circled.
The angled lines extending from the Ts indicate linkage with
the furan (F) ring or pyrone (P) ring of
TMP.
The 3` and 5` incisions were reduced (67 and 64%, respectively) upon the addition to the reaction mixture of 100 ng of unlabeled competitor DNA, which contained TMP furan side monoadducts (Fig. 5, lane 1; 3` and 5` incisions represent 4.9 and 2.8% of substrate, respectively, compared with lane 2, 3` and 5` incisions representing 1.6 and 1.0% of substrate, respectively). Nonspecific cuts were reduced by only 38% (Fig. 5, lane 1 compared with lane 2). Some reduction in nonspecific cuts would be expected since nonspecific nucleases would be acting on the competitor as well as on the substrate DNA. The dual incisions were totally abolished when 500 ng of the competitor was added (lane 4). In several of the figures, the DNA sequence around the damaged sites, when reacted with the protein extracts, appeared to be relatively protected. Since this pattern is seen on both damaged as well as on undamaged DNA (Fig. 4), it may represent sequence-specific protein binding rather than binding by a damage-specific binding protein, particularly since the 3` and 5` endonucleolytic incisions are seen only on the damaged and not on the undamaged DNA.
Figure 5:
Competitive inhibition of endonucleolytic
incisions on the furan side monoadducted substrate. The 132-bp P-labeled substrate was modified with TMP at a thymine
indicated by an asterisk. Maxam-Gilbert sequence reactions are
shown. Normal human chromatin-associated protein extracts (10 µg)
were incubated with the substrate in the presence or absence of
unlabeled modified DNA. The reaction products were analyzed on a
denaturing 6% polyacrylamide gel. The bands indicating incisions 3` and
5` to the adducted thymine are denoted by arrows. The pattern
of endonucleolytic incision on this substrate is as follows: lane
1, in the absence of competitor; lane 2, in the presence
of 100 ng of unlabeled TMP-adducted DNA; lane 3, in the
presence of 2000 ng of unlabeled competitor DNA; lane 4, in
the presence of 500 ng of unlabeled competitor
DNA.
For those substrates in which incisions at sites of interstrand cross-links were examined, the cross-linked DNA was photoreversed by UVC light before gel electrophoresis. This was done in order to break the interstrand cross-link and allow us to detect potential incisions on both sides of the lesion on the same strand in a single assay(35, 36) . Experiments were carried out to make sure that irradiation of the cross-linked DNA with UVC light was not affecting the electrophoretic migration pattern of the incised DNA. Monoadducted DNA, reacted with the human extract and irradiated with UVC light prior to electrophoresis, produced patterns of migration that were the same as when the DNA was not UVC-irradiated (data not shown). Therefore, irradiation with UVC light did not produce a shift in migration pattern of the DNA bands.
Examination of the ability of the normal extracts to incise DNA containing a furan side interstrand cross-link indicated that dual endonucleolytic incisions were also produced on this substrate (Fig. 4). Incisions occurred at the fourth phosphodiester bond 3` to the adducted thymine and at the sixth phosphodiester bond 5` to this modified base (Fig. 4, lane 2; 3` and 5` incisions, 4.4 and 2.8% of substrate, respectively). For both the furan side monoadduct and cross-link, the distance between sites of incision was 9 nucleotides. These same studies were carried out with similar extracts from HeLa cells, and the same results were obtained (data not shown).
Figure 6:
The
normal human endonucleolytic incision pattern on the 132-bp pyrone side
monoadducted substrate. Upper panel, the bottom strand, ML9,
was internally labeled with P. The TMP-modified thymine is
indicated by an asterisk. Maxam-Gilbert sequence reactions are
shown. Normal human chromatin-associated protein extracts were
incubated with the TMP pyrone side monoadducted substrate, and the
reaction products were analyzed on a denaturing 6% polyacrylamide gel.
Incisions on the 3` and 5` sides of the adducted thymine are indicated
by arrows. The TMP-modified substrate was reacted with 10 (lane 1) or 25 µg (lane 2) of the protein
extract. Undamaged substrate was treated with either 10 (lane
3) or 25 µg (lane 4) of the protein extract. Lower panel, sites of 3` and 5` endonucleolytic incision on
this substrate are indicated by arrows. Notations are the same
as in Fig. 4.
Dual incisions were also produced by the normal extracts on pyrone side cross-linked DNA. However, as noted above, these incisions were produced less efficiently than for the other TMP-adducted substrates. The 3` incision was at the fourth phosphodiester bond from the adducted thymine, and the 5` incision was at the 14th phosphodiester bond from this modified thymine (Fig. 7; lane 1, 3` and 5` incisions, 2.5 and 1.7% of substrate, respectively; lane 2, 3` and 5` incisions, 2.7 and 2.1% of substrate, respectively). The distance between sites of incision on both the pyrone side monoadducted and cross-linked substrates was 17 nucleotides, a greater distance than that between sites of incision on furan side adducted DNA.
Figure 7:
The normal human endonucleolytic incision
pattern on the 132-bp pyrone side cross-linked substrate. Upper
panel, the bottom strand, ML9, was internally labeled with P. The TMP-modified thymine is indicated by an asterisk. Maxam-Gilbert sequence reactions are shown. Normal
human chromatin-associated protein extracts were incubated with the TMP
pyrone side cross-linked substrate, and the reaction products were
analyzed on a denaturing 6% polyacrylamide gel. Sites of incision are
indicated by arrows. The pattern of incision on the
TMP-modified substrate reacted with 10 (lane 1) and 25 (lane 2) µg of the protein extract. Lane 3,
undamaged substrate treated with 10 µg of the protein extract. Lower panel, sites of 3` and 5` endonucleolytic incision on
this substrate are indicated by arrows. Notations are the same
as in Fig. 4.
All of the incision experiments described above were carried out using two different normal human cell lines. Each experiment was repeated 10-20 times using two to four different extractions from each cell line and three to four different substrate preparations. In all instances the results obtained were always the same. The addition of ATP was not required for any of these incision events.
Figure 8:
Ionic requirements for endonucleolytic
incision. The TMP furan side monoadducted 132-bp substrate was used for
this study. The TMP-modified thymine is indicated by an asterisk. Maxam-Gilbert sequence reactions are shown. Normal
human chromatin-associated protein extracts (10 µg) were reacted
with the TMP furan side monoadducted substrate, and the reaction
products were analyzed on a denaturing 6% polyacrylamide gel. The bands
indicating incisions 3` and 5` to the adducted thymine are denoted by
arrows. The enzymatic reactions were carried out in 50 mM Tris-HCl, pH 7.6, and one of the following: 10 mM
MgCl (lane 1), 2 mM MgCl
(lane 2), 0 mM MgCl
(lane
3), 10 mM MgCl
and 15 mM EDTA (lane 4), 10 mM MgCl
and 2 mM
Zn
(lane 5), 10 mM MgCl
and 2 mM Mn
(lane 6), or 2
mM Mn
(lane
7).
An important step in elucidating the mechanisms of action of human DNA repair proteins is to determine the events involved in the critical initial damage recognition and incision step in the repair process. Engineering and construction of DNA substrates of defined sequence and length containing a single site-directed lesion provide an excellent tool for investigation of the exact sites and precise nature of the incision events that can occur at such a lesion. Utilizing such a uniquely modified 132-bp substrate, which contained a single site-directed TMP adduct placed on a central thymine, we have directly examined the incision events involved in repair of psoralen plus UVA light-induced DNA monoadducts and interstrand cross-links by human chromatin-associated proteins. The results show that the human extracts produced incisions in DNA on both the 3` and 5` sides of each of the four types of TMP-thymine adducts. The distance between the 3` and 5` incisions for furan side monoadducts and cross-links was always 9 nucleotides, whereas the distance between these incisions for the pyrone side monoadducts and cross-links was always 17 nucleotides. This increased distance between sites of incision for the pyrone versus the furan side adducts could be due to the fact that, since TMP is an asymmetric molecule, more distortion may occur in the DNA in the vicinity of the thymine adducted to the pyrone side of TMP as compared with that of the thymine bound to the furan side(7, 9, 10, 37) . The sites of incision on the 3` side of both types of furan side and pyrone side adducts were similar; they were either at the fourth or fifth phosphodiester bond from the adducted thymine, depending upon the adduct (Fig. 9). Greater differences were observed, however, in the sites of incision on the 5` side. These incisions occurred at the fifth and sixth phosphodiester bonds from the adducted thymine at sites of furan side adducts and at the 13th and 14th phosphodiester bonds at sites of pyrone side adducts (Fig. 9). Thus, production of a potentially greater distortion in the DNA by the pyrone side adducts appears to have little effect on the site of the 3` incision but to increase markedly the distance of the 5` incision from the adducted thymine, compared with the distance of this incision from the site of furan side adducts.
Figure 9: Pattern of endonucleolytic incision by human chromatin-associated protein extracts on DNA containing TMP furan side and pyrone side monoadducts and interstrand cross-links. Arrows point to sites of incision.
Svoboda et al.(26) have examined removal of psoralen monoadducts by HeLa whole cell extracts. They utilized as substrate a plasmid DNA containing a single 4`-hydroxymethyl-4,5`8-trimethylpsoralen (HMT) furan side monoadduct. Based on their results using an excision assay, which examined the sizes of DNA fragments excised from HMT-monoadducted DNA after treatment with cell extracts(26) , and based on the pattern of excision of thymine dimers from DNA(26, 38) , they proposed that fragments of 27-32 nucleotides in length are excised and predicted that the sites of incision occur at the fourth or fifth phosphodiester bond 3` and the 22nd to 24th phosphodiester bond 5` to the adducted thymine. Their site of 3` incision is similar to the one we have observed, but their proposed site of 5` incision is significantly farther from the adducted thymine than ours. An important difference, however, between the methods utilized in the present study and those used by Svoboda et al. is that we are looking at sites of endonucleolytic incision directly and they used indirect methods of analysis. For their studies that examined the size of the DNA fragment excised, DNA was labeled at the eighth phosphodiester bond 5` to the HMT furan side adduct(26) . However, any fragment in which the 5` incision occurred between the seventh phosphodiester bond and the adduct would not have been detected in their system. Therefore, using this labeling scheme, Svoboda et al.(26) would not have been able to detect the 5` incision site we describe at the 5th phosphodiester bond from the adduct. Svoboda et al.(26) also predicted sites of incision for HMT monoadducts based on the excision pattern of thymine dimers. In these studies the boundaries of the repair patch synthesized, after excision of the damaged fragment, were examined and used in the determination of the sites of 3` and 5` incision(26, 38) . By using DNA fragments containing repair patches with phosphorothioate linkages and by analyzing resistance of these linkages to exonuclease III digestion and their sensitivity to cleavage by iodine, they delineated the borders of the repair patch(26, 38) . Since whole cell extracts were used for these studies, it is possible that specific as well as nonspecific endonucleolytic and exonucleolytic activities present in these extracts and postincision events could contribute to the size of the DNA fragment excised. The size of this fragment in turn determines the size of the repair patch generated. Thus, these events could account for the larger distance, reported by Svoboda et al.(26) , between the sites of the 3` and 5` incision. This is in contrast to results obtained when pure proteins are used in this type of excision assay, where a more direct correlation can be obtained between the size of the repair patch generated and the distance between sites of endonucleolytic incision, as has been shown for the UvrABC nuclease (39) .
Comparison of the incision pattern produced on psoralen-adducted substrates by the human endonucleases in the present study with that produced by the UvrABC nuclease(11, 12, 13, 14, 15, 16, 17) shows that the sites of 3` incision are fairly similar. Differences were observed, however, in the sites of 5` incision. The distance between sites of incision on all four types of psoralen-adducted substrates is 12 nucleotides for the UvrABC nuclease, compared with a distance of 9 and 17 nucleotides for the human endonucleases at sites of furan and pyrone side adducts, respectively. Also, in the present study, incisions at the pyrone side TMP cross-link were not produced as efficiently as those at the furan side cross-link. This result is analogous to those from the UvrABC system in which the pyrone side cross-linked substrate was not found to be efficiently incised in one study (14) and was not found to be incised at all in another(11) . Studies indicated that UvrABC incision on the pyrone side of the cross-linked substrate may depend upon whether or not a three-stranded intermediate is present(16, 17) . These incisions have also been shown to be sequence-dependent (14, 15) . It is possible that a similar situation may play a role in the incision of a pyrone side cross-linked substrate by the human repair endonuclease(s).
The present data show that incisions both 3` and 5` to a TMP adduct are produced on the 5`-labeled DNA substrate. This means that the 3` incision occurred in the absence of or independent of the 5` incision; otherwise, the 3` incision would not have been detected. This indicates that the 3` and 5` incision events are uncoupled. The 5` incision, in turn, could represent just a 5` incision event or the generation of a 5` incision accompanied by a 3` incision. Uncoupling of dual incisions at sites of DNA adducts has also been observed for the UvrABC nuclease at sites of pyrimidine dimers(40) , psoralen adducts(12, 15) , and CC-1065 adducts(41) .
The question arises whether the same or different proteins are involved in repair of the various different types of lesions that can be produced in DNA. The chromatin-associated protein extracts utilized in the present study contain an endonuclease complex, pI 4.6, which has specificity for DNA interstrand cross-links, and another, pI 7.6, which recognizes psoralen monoadducts(22, 23, 25, 42) . A damage recognition protein with specificity for interstrand cross-links, which we have identified in these extracts, is thought to be a component of the DNA endonuclease complex, pI 4.6(4) . These results are consistent with the hypothesis that different endonuclease complexes are involved in repair of psoralen monoadducts and interstrand cross-links(23, 25, 42) . It is, however, possible that the endonucleases involved in the production of the 3` and 5` incisions are the same in each complex but that each complex varies in the type of damage recognition protein present. On the other hand, different endonucleases could be associated with these complexes. The system described here, which specifically examines the initial incision events, combined with the use of human cell lines deficient in specific aspects of the damage recognition and incision step of the repair process, should greatly facilitate evaluation of this multifaceted and interactive process and of the proteins involved in these interactions.