Cellular strategies for accommodating replication-hindering adducts in DNA: control by the SOS response in Escherichia coli

Analysis of damage tolerance pathways in Saccharomyces cerevisiae: a requirement for Rev3 DNA polymerase in translesion synthesis

The replication of double-stranded plasmids containing a single N-2-acetylaminofluorene (AAF) adduct located in a short, heteroduplex sequence was analyzed in Saccharomyces cerevisiae. The strains used were proficient or deficient for the activity of DNA polymerase zeta (REV3 and rev3delta, respectively) in a mismatch and nucleotide excision repair-defective background (msh2delta rad10delta). The plasmid design enabled the determination of the frequency with which translesion synthesis (TLS) and mechanisms avoiding the adduct by using the undamaged, complementary strand (damage avoidance mechanisms) are invoked to complete replication. To this end, a hybridization technique was implemented to probe plasmid DNA isolated from individual yeast transformants by using short, 32P-end-labeled oligonucleotides specific to each strand of the heteroduplex. In both the REV3 and rev3delta strains, the two strands of an unmodified heteroduplex plasmid were replicated in approximately 80% of the transformants, with the remaining 20% having possibly undergone prereplicative MSH2-independent mismatch repair. However, in the presence of the AAF adduct, TLS occurred in only 8% of the REV3 transformants, among which 97% was mostly error free and only 3% resulted in a mutation. All TLS observed in the REV3 strain was abolished in the rev3delta mutant, providing for the first time in vivo biochemical evidence of a requirement for the Rev3 protein in TLS.

The Saccharomyces cerevisiae RAD30 gene, a homologue of Escherichia coli dinB and umuC, is DNA damage inducible and functions in a novel error-free postreplication repair mechanism

Damage-inducible mutagenesis in prokaryotes is largely dependent upon the activity of the UmuD'C-like proteins. Since many DNA repair processes are structurally and/or functionally conserved between prokaryotes and eukaryotes, we investigated the role of RAD30, a previously uncharacterized Saccharomyces cerevisiae DNA repair gene related to the Escherichia coli dinB, umuC and S. cerevisiae REV1 genes, in UV resistance and UV-induced mutagenesis. Similar to its prokaryotic homologues, RAD30 was found to be damage inducible. Like many S. cerevisiae genes involved in error-prone DNA repair, epistasis analysis clearly places RAD30 in the RAD6 group and rad30 mutants display moderate UV sensitivity reminiscent of rev mutants. However, unlike rev mutants, no defect in UV-induced reversion was seen in rad30 strains. While rad6 and rad18 are both epistatic to rad30, no epistasis was observed with rev1, rev3, rev7 or rad5, all of which are members of the RAD6 epistasis group. These findings suggest that RD30 participates in a novel error-free repair pathway dependent on RAD6 and RAD18, but independent of REV1, REV3, REV7 and RAD5.

SOS factors involved in translesion synthesis

Mutations are permanent DNA sequence changes that can be induced when replication occurs on a damaged DNA template. In Escherichia coli, the process of translesion synthesis past a lesion that hinders replication requires the induction of SOS-controlled gene products, among which are those of the umuDC operon. To study translesion synthesis in vivo, we have constructed single-stranded vectors containing single 2-acetylaminofluorene adducts located within -1 and -2 frameshift mutation hot spots formed by short repetitive sequences. These adducts strongly hinder DNA replication as only 2-5% of the molecules give rise to progeny under non-SOS-induced conditions. Induction of the SOS response lead to a 10-fold increase in survival. Adducts present within repetitive sequences trigger the formation of misaligned primer/template replication intermediates which, upon elongation, will result in the fixation of frameshift errors (mutagenic translesion synthesis). Surprisingly we find that elongation from the nonslipped intermediate depends upon functional umuDC+ gene products, whereas elongation from the slipped intermediate is umuDC+ independent but requires another, as yet biochemically uncharacterized, SOS function. These data are discussed in terms of the different steps involved during translesion synthesis through a replication-blocking lesion.

Leading versus lagging strand mutagenesis induced by 7,8-dihydro-8-oxo-2'-deoxyguanosine in Escherichia coli

We have previously shown that a single N-2-acetylaminofluorene (AAF) adduct bound to the C-8 position of a guanine residue located within plasmids containing the unidirectional ColE1 origin of replication induces a 20-fold higher mutation frequency when the adduct is located in the lagging strand as compared to the leading strand. In this study, single 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) lesions have been introduced in the leading and lagging strand orientation within the same sequence context as for the AAF adducts. The induced frequency of guanine to thymine transversions has been measured, using a specific PCR-based quantitative assay, in strains deficient in the repair of the oxidative lesion. The potential involvement of the UvrABC excision repair system in the removal of 8-oxodG has also been investigated and ruled out. Concerning the mutation frequency asymmetry, in contrast to AAF adducts, 8-oxodG adducts induce the same mutation frequency, irrespective of their location in the leading or lagging strands. This striking difference between 8-oxodG and dGuo-C8-AAF adducts is discussed in terms of their differential capacity to block DNA replication.