©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Conservation and Periodicity of DNA Bend Sites in the Human -Globin Gene Locus (*)

Yuko Wada-Kiyama (1)(§), Ryoiti Kiyama (2)

From the (1) Department of Physiology, Nippon Medical School, Sendagi, Bunkyo-ku, Tokyo 113 and the (2) Institute of Molecular and Cellular Biosciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113, Japan

ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES

ABSTRACT

A total of seven DNA bend sites were mapped in the 4.4-kilobase human -globin gene region by the circular permutation assay. The periodicity of these sites (except one) was about every 700 (average 685.5 ± 267.7) base pairs. All of the sites contained the sequence feature of short poly(dA) tracts, which are typical of DNA bending. The relative positions of the sites to the cap site were identical to those in the -globin gene region, suggesting that the bend sites were conserved during molecular evolution of the two globin genes. To explain this periodicity and conservation of the sites within the evolutionary unstable noncoding regions, we focused upon the appearance of a potential bend core sequence, ANANA (A/A/A), and its complement, TNTNT (T/T/T). These sequences appeared in or very close to most of the bend sites of the globin gene regions, whereas other A+T-rich sequences or candidates for DNA bending did not. The distances between any two of the core sequences in the entire -globin locus showed a strong bias to a length of about 700 base pairs and its multiples, suggesting that the periodicity exists throughout the locus. The data presented here strengthen the idea of sequence-directed nucleosome phasing.


INTRODUCTION

The human -globin locus consists of five active genes in the order of ---- on chromosome 11 (1, 2). These active genes and several pseudogenes, as well as the genes in the -globin locus, were considered to be derived from a single ancestral gene. The divergence of the ancestral globin gene first occurred about 500 million years ago (3, 4) . Since then, the number of the globin genes increased by a series of duplication and subsequent diversification, which enabled it to achieve the coordinated function of the globin gene family typically shown by switching of globin genes during embryonic development to accommodate oxygen transport under different environmental conditions. In the process of globin gene evolution, duplication created two identical genes, including the coding regions. Although the noncoding regions experienced extensive mutation and insertion of transposable elements, which resulted in the diversification of their nucleotide sequences, the coding regions retained their homology by the mechanism of gene conversion (5, 6) . The conservation of the coding regions can be explained by the mechanism that sequences that bear important functions are difficult to mutate and, therefore, are conserved during evolution.

Other than the coding regions, there are conserved sequences in the noncoding regions. Some of them are TATA, CAAT, and CAC boxes and the binding motifs for transcription factors such as GATA-1 (7) . Gumucio et al.(8) reported that 12 sequence motifs were conserved in the upstream -globin gene region in several species as protein binding sites (8) .

The sequence that confers DNA bending was first reported for kinetoplast DNA, and, since then, bent DNA has been identified in the genes of a wide variety of species (9) . The biological effects of the DNA bending were reported in conjunction with recombination, transcription, and replication (10, 11, 12, 13, 14, 15, 16, 17, 18, 19) . The molecular structure that causes DNA bending is not yet fully understood, but the unusual conformation adopted by repeated short poly(dA) tracts, especially when they are distributed in roughly 10-base intervals, shows the extensive bending characteristics (20, 21) .

We reported that DNA bend sites are located in a very organized manner, at an interval of about 700 bp,() in the noncoding region around the human -globin gene over 7 kb in length (22). The regularity of these bend sites in an evolutionary unstable noncoding region suggests that they are significant, possibly in the organization of chromatin structure. We, therefore, proposed that the sites act as a signal for nucleosome phasing (22) . We present here evidence that DNA bend sites have been conserved during molecular evolution of the genomic sequence, which strengthens their significance in the genomic organization as well as function.


EXPERIMENTAL PROCEDURES

Materials

Restriction enzymes were purchased from Takara (Kyoto) or New England Biolabs.

Plasmid Construct

Plasmids were constructed by subcloning a fragment from the plasmid containing the region between the positions -2282 (PstI) and 2161 (PstI) in pUC8 vector. The fragment in each plasmid was as follows: pBA41, -2256 (BamHI) to -1461 (BamHI); pDU, -1844 (DraI) to -1029 (DraI); pMHA30, -1444 (HincII) to -817 (HincII); pMHB12, -817 (HincII) to -79 (MscI); pBBA24, -406 (BstYI) to 321 (BstYI); pR65, -129 (RsaI) to 580 (RsaI); pH22, 403 (HaeIII) to 1300 (HaeIII); pHHc113, 403 (HaeIII) to 996 (DraI); pHE41, 918 (MslI) to 1300 (HaeIII); pDL7, 996 (DraI) to 1616 (DraI); pE81, 1394 (EcoRI) to 2161 (PstI); and pDES14, 1394 (EcoRI) to 1616 (DraI). Tandem duplicates of each fragment were inserted into the multiple cloning site of pBluescript SK(-).

Assay for DNA Bend Sites

The circular permutation assay for DNA bend sites originally described by Wu and Crothers (23) proceeded as described previously (22) . Briefly, about 1 µg of plasmid DNAs that contained duplicates of the regions of interest were linearized with the restriction enzymes shown (see Fig. 1 ). After mixing with an internal calibration marker (PvuII or AvaI digests of M13mp18) the DNAs were electrophoresed at 4 °C on 8% polyacrylamide gels (mono/bis = 19:1) in 50 mM Tris borate, 5 mM EDTA (TBE) buffer under the following conditions: 1 V/cm for 60-70 h for fragments of over 600 bp or 1.5 V/cm for 30-40 h for those smaller than 600 bp. Electrophoresis at 55 °C (3 V/cm for 7 h) was performed with at least one clone for each bend site to confirm that the bending was abolished at a high temperature.


Figure 1: Mapping of DNA bend sites in the human -globin gene region. a, the circular permutation assay performed at 4 or at 55 °C with the plasmid pDU, which contains the region between -1844 and -1029. U, unit length fragments; M, 422-bp AvaI fragments from M13mp18 DNA. b, summary of mapping. The plasmid DNA containing tandem duplicates of the region shown as a thickhorizontalline in each panel was digested with the enzymes shown below. The relative migration distance to the fastest migrating band was adjusted using marker DNA fragments (422-bp AvaI fragments or 322-bp PvuII fragments from M13mp18 DNA, whichever was closer to the unit fragments was used), which are shown in the panel. The verticalbar in the panel indicates the average thickness of the bands. The bend sites, tentatively and operatively defined as the regions between the second and third nearest restriction sites (see Table I), are shown as a shadowedbox. An additional bend site (B-2`) between B-2 and B-1 is also indicated in the figure. The restriction enzymes were: 1, AccI; 2, AciI; 3, AlwNI; 4, ApaI; 5, BamHI; 6, BfaI; 7, BglII; 8, BsaHI; 9, BsmI; 10, BsmAI; 11, Bsp1286I; 12BspHI; 13, BsrGI; 14BstXI; 15, BstYI; 16, DdeI; 17, DraI; 18, DraIII; 19, EarI; 20, EcoO109I; 21, EcoRI; 22, EcoT14I; 23, HincII; 24, HindIII; 25, HphI; 26, MaeIII; 27, MscI; 28, MslI; 29, MunI; 30, MvaI; 31, NcoI; 32, NlaIII; 33, NlaIV; 34, NspI; 35, Sau96I; 36, SnaBI; 37, SspI. The restriction sites derived from the cloning vector are in parentheses.



Computer Analysis

DNA sequences were analyzed by the program supplied by Software Development Co. using the sequence of the human -globin gene locus (73,326 bp, entry name HSHBB) or other sequences from the GenBank data bases.


RESULTS

Mapping DNA Bend Sites

Fig. 1 shows the results of mapping of the DNA bend sites in the human -globin gene region by the circular permutation assay (23, 24, 25) . First, clones containing tandem duplicates of a DNA fragment of about 500 bp to 1 kb were constructed. Then, the plasmid DNA was digested with a restriction enzyme that cuts the unit sequence once and thus produces a fragment with an identical length but in the permuted order of the unit sequence. The positions of the bend sites were detected by polyacrylamide gel electrophoresis at 4 °C as the fastest migrating band among the fragments created by various restriction enzymes, and the bend centers were likely to be located between the second and third nearest sites and close to the (first) nearest site. We defined the DNA bend sites as the region between the second and the third nearest sites (22). Fig. 1a shows the results of mapping of the bend site with pDU. In this assay, the bend site is likely to be located between EcoT14I and HincII and close to BglII. The anomaly of migration of restricted fragments detected at 4 °C was abolished at 55 °C, confirming that the anomaly was due to DNA bending (22) . A total of seven sites (B-3 to B+3 and B-2`) was mapped in the 5` and 3` regions as well as in the second intron (Fig. 1b). The positions of the bend sites relative to the canonical cap site were -1675 to -1461 for B-3, -1054 to -817 for B-2, -664 to -406 for B-2`, -373 to -210 for B-1, 714 to 918 for B+1, 996 to 1192 for B+2, and 1687 to 2032 for B+3, and the average interval between the sites (except B-2`) was 685.5 ± 267.7 bp. As shown in the figure, the presence of the bend sites in the middle of the fragments generally resulted in 7-20% retardation upon electrophoresis. When a bend site is absent in the fragment, however, all fragments of permuted sequences had identical mobility (see pR65). A clear bend site, B-2`, between -664 (Bsp1286I) and -406 (BstYI), mapped between the sites B-2 and B-1, was likely to be additional, deduced by the comparative alignment of the sites between the two globin gene regions (described below).

The positions of the bend sites are summarized in . Also shown is a comparison of the bend sites in the human -globin gene region (B-3 to B+3) (22) . The relative positions of the bend sites in the - and -globin gene regions were located close together, suggesting that these sites were conserved during the molecular evolution of the globin gene locus.

The sequences in the region of the bend sites are shown in Fig. 2. There were stretches of short poly(dA) tracts (shadowed in the figure) in all of them, which are typical of DNA bending. Other than this, there were no apparent sequence features in common among the sites.


Figure 2: Nucleotide sequences of the bend sites. Nucleotide sequences of the bend sites B-3 to B+3 were aligned in 10-nucleotide intervals to reveal the periodicity of short (at least three consecutive) poly(dA) tracts. The regions of potential bend centers were bracketed on the left.



Sequence Features of DNA Bend Sites

Since most of the bend sites in the - as well as -globin gene regions have three to more than 10 repeats of short (dA) (n 2) tracts with intervals of roughly 10, or multiples of 10 nucleotides, we focused upon the appearance of the short poly(dA) tracts with the consensus ANANA (A/A/A, and its complementary sequence TNTNT, T/T/T). Based on the observation that the DNA fragments with (dA)n (n 4) tracts at the interval of 10 nucleotides show a substantial bending feature (26) , these sequences are potentially bend core sequences. We chose these sequences because they include a wide variety of potentially bend core sequences. A more stringent screening, with ANANA for example, would exclude many bending sequences, such as ANANA, and would not provide a reliable result. As shown in Fig. 3a, the sequence A/A/A appeared roughly once every few hundred bases and fit well with the bend sites in the human -globin gene region. Especially, A/A/A sequences were found in the regions B-2, B-1, B+1, B+2 (see Fig. 3a), and B-2`. Other sequences known for bending, ANTN or GNCN(27) , did not show such periodicity (data not shown). Similarly, other combinations of the A or T dinucleotides in the 10-nucleotide interval, A/T/T (and A/A/T), T/T/A (and T/A/A), and A/T/A (and T/A/T), could not explain the bend sites (data not shown). Periodicity of the A/A/A sequence was also observed in the human -globin gene region (Fig. 3b).


Figure 3: The appearance of a potential bend core sequence A/A/A (and its complementary T/T/T) for DNA bending in the human - (a) and - (b) globin gene. A total of six bend sites both in the -globin gene (B-3 to B+3) and in the -globin gene (B-3 to B+3) are shown by shadowedboxes. The nearest A/A/A position for each bend site is shown by a filledcircle, and the A/A/A sequence, which is a part of poly(dA) sequences, is shown by an opencircle.



Periodicity of A/A/A Sequences in Human -Globin Locus

To examine the periodicity of the A/A/A sequence in the entire human -globin locus, we searched for the sequence in more than 70 kb of the locus and scored the distance between any two of the sequences. Fig. 4a shows the distribution of the distances in the range of 1-2000 bp. As shown in the figure, there appeared a biased distribution: two major peaks centered at 701-800 and 1301-1400 bp. Since the consensus A/A/A has variations that actually cannot confer bending capability because it is too A+T-rich, such as A(A)A(A)A (all Ns are As), we subtracted the ANANA sequences where the total number of A and T in the total of 16 Ns exceeded nine (referred to as A+T 9/16, Fig. 4b). This subtraction was also justified by the fact that most of the A+T-rich sequences appeared as the consensus A/A/A were a mix of adenines and thymines or a poly(dA) tail of retroposons or pseudogenes, instead of having a characteristic of long consecutive adenines or thymines such as (AN) or (AN), which also have a bending characteristic (data not shown). More strict conditions, A+T 8/16 for example, showed similar results, although the total numbers of the sites were not enough for statistical analysis. As shown in the figure, the two peaks that appeared in Fig. 4a remained predominant. This tendency was indicated as a deviation from a random distribution by a simple calculation; the total frequency of 701-800 bp and 1301-1400 bp divided by the total incidents, was then normalized by multiplying by 10. If the distribution is random, the value should be 1.00. Actually, the real value was less than 1.00 because of the tendency of A+T-rich sequences to cluster. The value for the -globin gene locus was 1.36, and that for the A/A/A where A+T 9/16 was 1.68. This indicated that the appearance of the A/A/A sequences, especially less A+T-rich ones, was biased toward a periodicity of about 700 bp and its multiples, which explains the periodicity of the bend sites in both globin gene regions. Furthermore, as shown in Fig. 4c, the base distribution in the variations of the A/A/A (A+T 9/16) sequences exhibited the following features that enhance bending efficiency (21) : the preference for a longer A stretch (N7 and N8 for A, resulting in 3-4 consecutive As), interruption of the A stretches by G (N1, N1`, and N8` for G) and the preference of a T base in the middle (N3, N5, N3`, and N5`). We also noted that there were peaks at 1001-1100 and 1701-1800 bp, located in the middle of the peaks of multiples of about 700 bp. A similar survey of human and other eukaryotic genomes, eukaryotic mRNAs, and Escherichia coli and viral genomes revealed that the bias toward multiples of about 700 bp is universal among eukaryotic genomes (data not shown). On the other hand, the eukaryotic mRNAs or E. coli and viral genes had no such bias.


Figure 4: Distribution of the distances between any two of the A/A/A sequences in the range of 1-2000 bp in the human -globin locus. a, a survey of all A/A/A sequences. b, a survey of the A/A/A sequences where A+T 9/16. The positions of 701-800 bp and 1301-1400 bp are indicated by filledarrows, while those of 1001-1100 bp and 1701-1800 bp are indicated by openarrows. Two or more A/A/A sequences appearing within 30 bp are represented by the one in the middle or the mean. c, the relative appearance (in %) of each base in the A/A/A sequences (A+T 9/16). The predominant (at least 5% higher than others) bases are shadowed.



Sequence Features for DNA Bending at B-2 and B-2

To examine whether the A/A/A sequences are responsible for the observed DNA bending, we focused upon these sequences appeared at B-2 and B-2. These sites were selected because both sites are well conserved between - and -globin genes and were accompanied by the A/A/A sequences within the sites. We constructed a total of four serial deletions encompassing the B-2 regions for each globin gene, and the relative migration was compared between the BamHI site, which was introduced at both ends (the common upstream positions and the downstream positions N, S, M, or L) and used as a control site, and the ApaI (for -globin) or HphI (for -globin) sites, which should show a bending characteristic if a bend site is included (Fig. 5a). We took this approach because in the circular permutation assay, the locations of other regions that potentially affect DNA bending are also permuted, and mapping is totally dependent upon the availability of appropriate restriction sites, both of which complicate the mapping process. As shown in Fig. 5, b and c, when a part of the B-2 sites (S, M, and L sites for -globin, and M and L sites for -globin) was present, DNA bending was significantly recovered. In both cases, although relatively large regions were required for a full-scale bending characteristic, the regions containing the A/A/A sequences (underlined in the lowerpart of Fig. 5a) seemed to be partly responsible for the DNA bending. This was confirmed by the bending assay with concatenated oligonucleotides with the A/A/A sequences from these regions (Fig. 5, d and e), which indicates that the fragments with these A/A/A sequences actually bend.


Figure 5: Fine mapping of DNA bend sites at B-2 sites of - and -globin genes. a, maps of the clones containing the B-2 regions. The downstream positions (N, S, M, or L) of the deletion constructs are indicated. Deletion constructs (clones pN1-9, -1384 to -1097, pS2-4, -1384 to -1047, pM3-36, -1384 to -997, and pLa-16, -1384 to -947, for -globin; pN4-7, -1367 to -985, pS5-60, -1367 to -935, pM6-33, -1367 to -885, and pLb-13, -1367 to -835, for -globin) containing tandem duplicates of the indicated region (unit length fragment) were created by polymerase chain reaction using 28-nucleotide primers containing the BamHI site (GGATCCGC) at the 3` end and cloned into the BamHI site of pBluescript SK(-). Therefore, each deletion construct has the BamHI site at the ends and produces the unit length fragments by this enzyme. The A/A/A sequences (locations -1056 to -1015 for -globin and -944 to -923 for -globin) are indicated by shadedboxes (upper) and underlined (lower). The regions used for the bending assay with concatenated oligonucleotides are bracketed at the bottom. b, the bending assay with the clones containing deletions. Approximately 0.5 µg of plasmid DNA was digested with BamHI for control fragments (C), or ApaI (for -globin) or HphI (for -globin) for bend fragments (B) and electrophoresed on 8% polyacrylamide gels at 4 °C for 48 h. BamHI was replaced by BfaI for pMHA30 (see Fig. 1b) and ClaI for p200LE2III (22). The unit length fragments are indicated by dots (the sizes are shown in a). c, the relative migration of the bend fragments to the control ones was calculated and plotted for each deletion construct (N, S, M, and L) and the original clones (All). The assay was repeated twice, and the standard deviations are indicated. d, the bending assay with concatenated oligonucleotides. The 20-base-long oligonucleotides containing the A/A/A sequences (-1056 to -1037, A+T and, -944 to -925, A+T) and the control oligonucleotides, 20-nucleotide-long poly(dA) and poly(dT) (A+T), which show a normal migration (see Ref. 26), were annealed and ligated as described previously (22) and then electrophoresed on an 8% polyacrylamide gel at 4 °C for 23 h. e, the R values (ratio of apparent length to real length, see Ref. 26) for the A/A/A sequences (A+T and A+T) obtained from d. The control A+T was used as a size standard.




DISCUSSION

We previously reported the periodic appearance of DNA bend sites in the noncoding region of the human -globin gene. We identified a total of 10 bend sites in the 7-kb region (from -4615 to +2382 relative to the cap site of the -globin gene), which appeared every 680 bp on average (22) . Similar findings in the 5` flanks of other eukaryotic genes prompted us to investigate the presence and the positions of the sites in the human -globin gene region. As summarized in Fig. 1, a total of seven bend sites were mapped in the region. The relative positions of the six sites, B-3 to B+3, to the cap site were strikingly similar to those in the -globin gene region, and the average distance between the sites was also about 700 bp in length. This raised the question as to how those sites were conserved during evolution and why.

The human - and -globin genes were separated about 200 million years ago, and, since then, although the coding regions (exons) retained sequence homology, random mutations in the noncoding regions (introns and 5`- and 3`-flanks) resulted in sequences with almost no homology (3) . This was typically shown by Harr-plot analysis, where no stretches of conserved sequences, as judged by the presence of perfect matches eight or nine nucleotides long, were identified in the noncoding regions around both globin genes (data not shown). Furthermore, analysis under less strict conditions, eight matches out of 10 nucleotides for example, also failed to reveal sequence conservation between the regions (data not shown). There is an Alu family sequence between the sites B-3 and B-2 in the -globin gene region (see Fig. 3). Although the sequence is absent in the -globin gene region (between B-3 and B-2), the distance between the sites was conserved, suggesting that the structure of DNA bending rather than the nucleotide sequence of the bend sites was conserved during evolution. It is, therefore, natural to assume that conservation of the bend sites should be caused by the biological significance of the sites.

We previously proposed that the bend sites that appear at a 700-bp interval are used as a signal for nucleosome phasing and can facilitate efficient and accurate folding of the chromatin structure during chromosome condensation (22) . Although it is still a matter of discussion whether DNA bending actually occurs in vivo, the A+T-rich sequences tend to be excluded from the nucleosomes and, therefore, could phase the nucleosomes and facilitate the chromatin folding.

Our present findings suggest that the periodicity of the bend sites is mainly directed by the regions that include the sequences containing a consensus A/A/A and that the periodicity is universal among eukaryotic genomes. Periodicity, or long range correlation of the nucleotide bases among eukaryotic genomes have been reported by several groups (28, 29, 30, 31, 32, 33) . Peng et al.(32) suggested long range correlation of the nucleotide sequences in genes with introns, and the correlation was absent in the genes without introns or mRNAs (32) . The periodicity of nucleotide bases has also been shown by digesting genomic DNA with less base-specific nucleases such as DNase I (33) . From the results of in vitro reconstitution of nucleosomes, van Holde and co-workers (34, 35) postulated that nucleosome positioning is an inherent property of nucleotide sequences. It also has been shown that nucleosomes could be phased by DNA sequences containing the dinucleotide A/T or trinucleotide A/T(36, 37) . Therefore, together with the fact that the bend sites in the intergenic regions have been maintained throughout genome evolution, the periodicity of the bend sites in eukaryotic genomes would not be a result of the nucleosome phasing but instead could be actively involved in forming and arranging the nucleosomes.

  
Table: Comparison of the bend sites between the human - and -globin gene regions



FOOTNOTES

*
This work was supported by grants-in-aid from the Ministry of Education of Japan. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§
To whom correspondence should be addressed: Dept. of Physiology, Nippon Medical School, Sendagi, Bunkyo-ku, Tokyo 113, Japan. Tel.: 81-3-3822-2131; Fax: 81-3-5685-3055.

The abbreviations used are: bp, base pair(s); kb, kilobase(s).


REFERENCES
  1. Collins, F. & Weissman, S.(1984) Prog. Nucleic Acids Res. Mol. Biol. 32, 315-462
  2. Stamatoyannopoulos, G., Nienhuis, A. W., Leder, P. & Majerus, P. W. (eds)(1987) The Molecular Basis of Blood Diseases, Saunders, Philadelphia, PA
  3. Efstatiadis, A., Posakony, J. W., Maniatis, T., Lawn, R. M., O'Connell, C., Spritz, R. A., DeRiel, J. K., Forget, B. G., Weissman, S. M., Slightom, J. L., Blechl, A. E., Smithies, O., Baralle, F. E., Shoulders, C. C. & Proudfoot, N. J.(1980) Cell 21, 653-668 [Medline] [Order article via Infotrieve]
  4. Proudfoot, N. J., Gil, A. & Maniatis, T.(1982) Cell 31, 553-563 [Medline] [Order article via Infotrieve]
  5. Slightom, J. L., Blechl, A. E. & Smithies, O.(1980) Cell 21, 627-638 [Medline] [Order article via Infotrieve]
  6. Maeda, N., Bliska, J. B. & Smithies, O.(1983) Proc. Natl. Acad. Sci. U. S. A. 80, 5012-5016 [Abstract]
  7. Grosveld, F., Antoniou, M., Blom van Assendelft, G., Catala, F., Collis, P., deBoer, E., Dillon, N., Greaves, D. R., Hanscombe, O., Hurst, J., Lindenbaum, M., Spanopoulou, E., Talbot, D. & Wall, L. (1989) The Regulation of the Human -globin Domain, in Tissue-specific Gene Expression. (Renkawitz, R., ed) VCH Publishers, Weinheim, Germany
  8. Gumucio, D. L., Heilstedt-Williamson, H., Gray, T. A., Tarle, S. A., Shelton, D. A., Tagle, D. A., Slightom, J. L., Goodman, M. & Collins, F. S.(1992) Mol. Cell. Biol. 12, 4919-4929 [Abstract]
  9. Crothers, D. M., Haran, T. E. & Nadeau, J. G.(1990) J. Biol. Chem. 265, 7093-7096 [Free Full Text]
  10. Goodman, S. D. & Nash, H. A.(1989) Nature 341, 251-254 [CrossRef][Medline] [Order article via Infotrieve]
  11. Bossi, L. & Smith, D. M.(1984) Cell 39, 643-652 [Medline] [Order article via Infotrieve]
  12. McAllister, C. F. & Achberger, E. C.(1988) J. Biol. Chem. 263, 11743-11749 [Abstract/Free Full Text]
  13. Wolffe, A. P. & Drew, H. R.(1989) Proc. Natl. Acad. Sci. U. S. A. 86, 9817-9821 [Abstract]
  14. Bracco, L., Kotlarz, D., Kolb, A., Diekmann, S. & Buc, H.(1989) EMBO J. 8, 4289-4296 [Abstract]
  15. Collis, C. M., Molloy, P. L., Both, G. W. & Drew, H. R.(1989) Nucleic Acids Res. 17, 9447-9468 [Abstract]
  16. Owen-Hughes, T. A., Pavitt, G. D., Santos, D. S., Sidebotham, J. M., Hulton, C. S., Hinton, J. C. D. & Higgins, C. F.(1992) Cell 71, 255-265 [Medline] [Order article via Infotrieve]
  17. Snyder, M., Buchman, A. P. & Davis, R. W.(1986) Nature 326, 87-89
  18. Zahn, K. & Blattner, F. R.(1987) Science 236, 416-422 [Medline] [Order article via Infotrieve]
  19. Williams, J. S., Eckdahl, T. T. & Anderson, J. N.(1988) Mol. Cell. Biol. 8, 2763-2769 [Medline] [Order article via Infotrieve]
  20. Travers, A. A.(1989) Annu. Rev. Biochem. 58, 427-452 [CrossRef][Medline] [Order article via Infotrieve]
  21. Hagerman, P. J.(1990) Annu. Rev. Biochem. 59, 755-781 [CrossRef][Medline] [Order article via Infotrieve]
  22. Wada-Kiyama, Y. & Kiyama, R.(1994) J. Biol. Chem. 269, 22238-22244 [Abstract/Free Full Text]
  23. Wu, H.-M. & Crothers, D. M.(1984) Nature 308, 509-513 [Medline] [Order article via Infotrieve]
  24. Stenzel, T. T., Patel, P. & Bastia, D.(1987) Cell 49, 709-717 [Medline] [Order article via Infotrieve]
  25. Schroth, G. P., Siino, J. S., Cooney, C. A., Th'ng, J. P. H., Ho, P. S. & Bradbury, E. M.(1992) J. Biol. Chem. 267, 9958-9964 [Abstract/Free Full Text]
  26. Koo, H.-S., Wu, H.-M. & Crothers, D. M.(1986) Nature 320, 501-506 [Medline] [Order article via Infotrieve]
  27. Shrader, T. E. & Crothers, D. M.(1989) Proc. Natl. Acad. Sci. U. S. A. 86, 7418-7422 [Abstract]
  28. Bodnar, J. W. & Ward, D. C.(1987) Nucleic Acids Res. 15, 1835-1851 [Abstract]
  29. J. Filipski, J., Leblanc, J., Youdale, T., Sikorska, M. & Walker, P. R. (1990) EMBO J. 9, 1319-1327 [Abstract]
  30. Walker, P. R., Sikorska, M. & Whitfield, J. F.(1986) J. Biol. Chem. 261, 7044-7051 [Abstract/Free Full Text]
  31. Constanzo, G., Di Mauro, E., Salina, G. & Negri, R.(1990) J. Mol. Biol. 216, 363-374 [Medline] [Order article via Infotrieve]
  32. Peng, C.-K., Buldyrev, S. V., Goldberger, A. L., Havling, S., Sciortino, F., Simons, M. & Stranley, H. E.(1992) Nature 356, 168-170 [CrossRef][Medline] [Order article via Infotrieve]
  33. Hutchison, H. & Weintraub, H.(1985) Cell 43, 471-482 [CrossRef][Medline] [Order article via Infotrieve]
  34. Dong, F., Hansen, J. C. & van Holde, K. E.(1990) Proc. Natl. Acad. Sci. U. S. A. 87, 5724-5728 [Abstract]
  35. Hansen, J. C., van Holde, K. E. & Lohr, D.(1991) J. Biol. Chem. 266,4276-4282 [Abstract/Free Full Text]
  36. Ioshikhes, I., Bolshoy, A. & Trifonov, E. N.(1992) J. Biomol. Struct. & Dyn. 9, 1111-1117
  37. Muyldermans, S. & Travers, A. A.(1994) J. Mol. Biol. 235, 855-870 [CrossRef][Medline] [Order article via Infotrieve]

©1995 by The American Society for Biochemistry and Molecular Biology, Inc.