Roles of DNA Polymerase ζ and Rev1 Protein in Eukaryotic Mutagenesis and Translesion Replication

Excerpt

In addition to mechanisms that specifically repairDNA, such as those that excise damaged nucleotides orbases, eukaryotes possess several processes that promotethe tolerance of unrepaired DNA damage. These processes are concerned with overcoming a potentially lethalproperty of unrepaired template damage, namely, itspropensity to block or severely impede DNA replication.One of the strategies used to overcome this inhibition istranslesion replication, in which extension of the nascentstrand at blocked forks is promoted by means of specialized DNA polymerases that are presumably more tolerantof alterations in primer/template structure than the normal replicases. In budding yeast, the eukaryote in whichtolerance mechanisms have been most intensively studied, the major process for translesion replication employsproducts encoded by the REV1, REV3, and REV7 genes(Lemontt 1971; Lawrence et al. 1985a,b). Rev3p is thecatalytic subunit of DNA polymerase ζ (pol-ζ) (Morrisonet al. 1989; Nelson et al. 1996a), a distant relative of DNApolymerase δ (Braithwaite and Ito 1993), and Rev7p isanother subunit of pol-ζ (Torpey et al. 1994; Nelson et al.1996a). Rev1p contains domains found in a superfamilythat includes the Escherichia coli UmuC and DinB geneproducts, the DinB homolog encoded by yeast RAD30,and the products of several mammalian RAD30 homologs(for review, see Gerlach et al. 1999; Woodgate 1999). Although the process employing the pol-ζ and Rev1p enzymes provides only a modest fraction of the capabilityof yeast to tolerate unrepaired damage, the activities ofthese enzymes are a major source of both spontaneousand induced mutations. For the most part, pol-ζ andRev1p are responsible for the production of base-pairsubstitutions and single nucleotide frameshifts, ratherthan larger additions or deletions, and their activities areusually the principal source of single nucleotide alterations. Greater than 98% of the base-pair substitutions induced by UV-irradiation, and more than 90% of the UVinduced frameshift mutations depend on the function ofpol-ζ, for example (Lawrence and Christensen 1979;Lawrence et al. 1984). Rev1p function is responsible formore than 95% of UV-induced base-pair substitutions,but appears to have less involvement in the induction offrameshift mutations, at least at some genetic sites(Lawrence and Christensen 1978; Lawrence et al. 1984).Although less well investigated, the mutagenic effectiveness of other mutagens such as ionizing radiation (McKeeand Lawrence 1979) and chemical agents (Lawrence etal. 1985b) also depend substantially on pol-ζ and Rev1pfunction. In addition, pol-ζ is responsible for between50% and 75% of spontaneous mutations (Quah et al.1980); for the increased spontaneous mutation resultingfrom transcription (Datta and Jinks-Robertson 1995), recombination (Holbeck and Strathern 1995), and the overexpression of 3-methyl adenine glycosylase (Glassner etal. 1998); and for a substantial fraction of the enhancedspontaneous mutagenesis in an excision-defective strain(Roche et al. 1994). Genetic evidence suggests that, asidefrom translesion replication, pol-ζ and Rev1p have nopart in any other replication, recombination, or repair activity. Recent evidence suggests that yeast possesses asecond translesion pathway, which employs DNA polymerase η (pol-η) (Johnson et al. 1999), encoded byRAD30 (McDonald et al. 1997). A comparison betweenthe phenotypes of RAD30 and REV mutants indicates thatpol-η has a relatively restricted range of substrates...