Sequence specificity of pausing by DNA polymerases

Analysis of replication intermediates indicates that Drosophila melanogaster mitochondrial DNA replicates by a strand-coupled theta mechanism

Mitochondrial DNA synthesis is necessary for the normal function of the organelle and for the eukaryotic organism as a whole. Here we demonstrate, using two-dimensional agarose gel electrophoresis to analyse replication intermediates, that unidirectional, strand-coupled DNA synthesis is the prevalent mode of mtDNA replication in Drosophila melanogaster. Commencing within the single, extended non-coding region (NCR), replication proceeds around the circular genome, manifesting an irregular rate of elongation, and pausing frequently in specific regions. Evidence for a limited contribution of strand-asynchronous DNA synthesis was found in a subset of mtDNA molecules, but confined to the ribosomal RNA gene region, just downstream of the NCR. Our findings imply that strand-coupled replication is widespread amongst metazoans, and should inform future research on mtDNA metabolism in D. melanogaster.

Highly mutagenic replication by DNA polymerase V (UmuC) provides a mechanistic basis for SOS untargeted mutagenesis

When challenged by DNA-damaging agents, Escherichia coli cells respond by inducing the SOS stress response, which leads to an increase in mutation frequency by two mechanisms: translesion replication, a process that causes mutations because of misinsertion opposite the lesions, and an inducible mutator activity, which acts at undamaged sites. Here we report that DNA polymerase V (pol V; UmuC), which previously has been shown to be a lesion-bypass DNA polymerase, was highly mutagenic during in vitro gap-filling replication of a gapped plasmid carrying the cro reporter gene. This reaction required, in addition to pol V, UmuD', RecA, and single-stranded DNA (ssDNA)-binding protein. pol V produced point mutations at a frequency of 2.1 x 10(-4) per nucleotide (2.1% per cro gene), 41-fold higher than DNA polymerase III holoenzyme. The mutational spectrum of pol V was dominated by transversions (53%), which were formed at a frequency of 1.3 x 10(-4) per nucleotide (1. 1% per cro gene), 74-fold higher than with pol III holoenzyme. The prevalence of transversions and the protein requirements of this system are similar to those of in vivo untargeted mutagenesis (SOS mutator activity). This finding suggests that replication by pol V, in the presence of UmuD', RecA, and ssDNA-binding protein, is the basis of chromosomal SOS untargeted mutagenesis.

Herpes simplex virus processivity factor UL42 imparts increased DNA-binding specificity to the viral DNA polymerase and decreased dissociation from primer-template without reducing the elongation rate

Herpes simplex virus DNA polymerase consists of a catalytic subunit, Pol, and a processivity subunit, UL42, that, unlike other established processivity factors, binds DNA directly. We used gel retardation and filter-binding assays to investigate how UL42 affects the polymerase-DNA interaction. The Pol/UL42 heterodimer bound more tightly to DNA in a primer-template configuration than to single-stranded DNA (ssDNA), while Pol alone bound more tightly to ssDNA than to DNA in a primer-template configuration. The affinity of Pol/UL42 for ssDNA was reduced severalfold relative to that of Pol, while the affinity of Pol/UL42 for primer-template DNA was increased approximately 15-fold relative to that of Pol. The affinity of Pol/UL42 for circular double-stranded DNA (dsDNA) was reduced drastically relative to that of UL42, but the affinity of Pol/UL42 for short primer-templates was increased modestly relative to that of UL42. Pol/UL42 associated with primer-template DNA approximately 2-fold faster than did Pol and dissociated approximately 10-fold more slowly, resulting in a half-life of 2 h and a subnanomolar Kd. Despite such stable binding, rapid-quench analysis revealed that the rates of elongation of Pol/UL42 and Pol were essentially the same, approximately 15 [corrected] nucleotides/s. Taken together, these studies indicate that (i) Pol/UL42 is more likely than its subunits to associate with DNA in a primer-template configuration rather than nonspecifically to either ssDNA or dsDNA, and (ii) UL42 reduces the rate of dissociation from primer-template DNA but not the rate of elongation. Two models of polymerase-DNA interactions during replication that may explain these findings are presented.

Cloning, characterization, and properties of seven triplet repeat DNA sequences

Several neuromuscular and neurodegenerative diseases are caused by genetically unstable triplet repeat sequences (CTG.CAG, CGG.CCG, or AAG.CTT) in or near the responsible genes. We implemented novel cloning strategies with chemically synthesized oligonucleotides to clone seven of the triplet repeat sequences (GTA.TAC, GAT.ATC, GTT.AAC, CAC.GTG, AGG.CCT, TCG.CGA, and AAG.CTT), and the adjoining paper (Ohshima, K., Kang, S., Larson, J. E., and Wells, R. D.(1996) J. Biol. Chem. 271, 16784-16791) describes studies on TTA.TAA. This approach in conjunction with in vivo expansion studies in Escherichia coli enabled the preparation of at least 81 plasmids containing the repeat sequences with lengths of approximately 16 up to 158 triplets in both orientations with varying extents of polymorphisms. The inserts were characterized by DNA sequencing as well as DNA polymerase pausings, two-dimensional agarose gel electrophoresis, and chemical probe analyses to evaluate the capacity to adopt negative supercoil induced non-B DNA conformations. AAG.CTT and AGG.CCT form intramolecular triplexes, and the other five repeat sequences do not form any previously characterized non-B structures. However, long tracts of TCG.CGA showed strong inhibition of DNA synthesis at specific loci in the repeats as seen in the cases of CTG.CAG and CGG.CCG (Kang, S., Ohshima, K., Shimizu, M., Amirhaeri, S., and Wells, R. D.(1995) J. Biol. Chem. 270, 27014-27021). This work along with other studies (Wells, R. D.(1996) J. Biol. Chem. 271, 2875-2878) on CTG.CAG, CGG.CCG, and TTA.TAA makes available long inserts of all 10 triplet repeat sequences for a variety of physical, molecular biological, genetic, and medical investigations. A model to explain the reduction in mRNA abundance in Friedreich's ataxia based on intermolecular triplex formation is proposed.

The chicken beta-globin gene promoter forms a novel "cinched" tetrahelical structure

We have previously shown that the G-rich sequence G16CG(GGT)2GG in the promoter region of the chicken beta-globin gene poses a formidable barrier to DNA synthesis in vitro (Woodford et al., 1994, J. Biol. Chem. 269, 27029-27035). The K+ requirement, template-strand specificity, template concentration independence, and involvement of Hoogsteen bonding suggested that the underlying basis of this new type of DNA synthesis arrest site might be an intrastrand tetrahelical structure. However, the arrest site lacks the four G-rich repeats that are a hallmark of previously described intramolecular tetraplexes and contains a number of noncanonical bases that would be expected to greatly destabilize such a structure. Here we report evidence for an unusual K+-dependent intrastrand "cinched" tetraplex. This structure has several unique features including the incorporation of bases other than guanine into the stem of the tetraplex, interaction between loop bases and bases in the flanking region, and base pairing between bases 3 and 5 of the tetrahelix-forming region to form a molecular "cinch." This finding extends the range of sequences capable of tetraplex formation as well as our appreciation of the conformational complexity of the chicken beta-globin promoter.

Inhibition of Klenow fragment DNA polymerase on double-helical templates by oligonucleotide-directed triple-helix formation

We have examined the capacity of oligonucleotide-directed triple helices to block the progress of primer extension by DNA polymerase. Occupancy of the major groove of a double-helical DNA substrate obstructed Klenow fragment progress at sites that map near the proximal boundary between duplex and triplex. Among a family of related third-strand oligonucleotides that all stably occupied the target duplex in the absence of polymerase, those forming longer triplexes were more effective polymerase inhibitors than shorter complexes. Kinetic analysis revealed that the triple-helical complex provided an effective blockade for times of at least 20 min. These observations provide the basis for considering and further dissecting repair DNA polymerase function and mechanism by using various defined local three-stranded DNA structures as probes.

Purification and characterization of p27, a protein from hepatocyte chromatin. Evidence suggesting that it binds selectively to guanine-rich single-stranded DNA

A protein designated p27 that binds preferentially to single-stranded DNA rich in guanine tracts was purified to near homogeneity from rabbit hepatocyte non-histone protein extract. Purified p27 migrated as a 27 kDa polypeptide on denaturing SDS-PAGE and displayed a native molecular mass of approximately 155 kDa on Sephadex G-150 or Sepharose 6B-Cl gel filtration columns. Gel shift analysis indicated that maximum binding of p27 to single-stranded DNA required the presence of tracts of four or more contiguous guanine residues. The lowest found dissociation constant, 1.4 x 10(-8) M/l, was for single-stranded DNA that contained a (dG)17 run.

A DNA polymerase alpha pause site is a hot spot for nucleotide misinsertion

In this study we examined whether the arrest of DNA polymerase alpha (pol alpha)-catalyzed DNA synthesis at template pause sites entails terminal nucleotide misincorporation. An approach was developed to identify the 3'-terminal nucleotide in nascent DNA chains that accumulate at pause sites. A radioactive 5'-end-labeled primer was annealed to a bacteriophage M13mp2 single-stranded DNA template and elongated by pol alpha. Individual DNA chains that were accumulated at pause sites were resolved by sequencing gel electrophoresis, isolated, and purified. These DNA chains were elongated by pol alpha by using four annealed synthetic DNA templates, each of which contained a different nucleotide at the position opposite the 3' terminus of the arrested chain. Owing to the high preference of pol alpha for matched-over-mismatched primer termini, only those templates that contain a nucleotide that is complementary to the 3' terminus of the isolated pause-site chain are copied. Electrophoresis of product DNA showed the extent of copying of each template and thus identified the 3'-terminal nucleotide of the pause-site chains. We found that product DNA chains terminate with a noncomplementary 3'-terminal nucleotide opposite pause sites within the sequence 3'-d(AAAA)-5' at positions 6272-6269 of the M13mp2 genome. pol alpha catalyzed misincorporation of dG or dA into the 3' terminus of nascent chains opposite two of the M13mp2 template dA residues. A similar analysis of a different pause site did not reveal significant misincorporation opposite template dC. These results suggest that some but not all sites at which pol alpha pauses may constitute loci of mutagenesis.

Limits to the role of palindromy in deletion formation

We tested the effect of palindromy on deletion formation. This involved a study of reversion of insertion mutations in the pBR322 amp gene at a site where deletions end either in 9-bp direct repeats or in adjoining 4-bp direct repeats. Inserts of palindromic DNAs ranging from 10 to more than 26 bp and related nonpalindromic DNAs were compared. The frequency of deletions (selected as Ampr revertants) was stimulated by palindromy only at lengths greater than 26 bp. The 4-bp direct repeats, one component of which is located in the palindromic insert, were used preferentially as deletion endpoints with palindromes of at least 18 bp but not of 16 or 10 bp. We interpret these results with a model of slippage during DNA replication. Because deletion frequency and deletion endpoint location depend differently on palindrome length, we propose that different factors commit a molecule to undergo deletion and determine exactly where deletion endpoints will be.