Nature of the SOS mutator activity: Genetic characterization of untargeted mutagenesis in Escherichia coli

Abstract

In Escherichia coli, induction of the SOS functions by UV irradiation or by mutation in the recA gene promotes an SOS mutator activity which generates mutations in undamaged DNA. Activation of RecA protein by the recA730 mutation increases the level of spontaneous mutation in the bacterial DNA. The number of recA730-induced mutations is greatly increased in mismatch repair deficient strains in which replication errors are not corrected. This suggests that the majority of recA730-induced mutations (90%) arise through correctable, i.e. non-targeted, replication errors. This recA730 mutator effect is suppressed by a mutation in the umuC gene. We also found that dam recA730 double mutants are unstable, segregating clones that have lost the dam or the recA mutations or that have acquired a new mutation, probably in one of the genes involved in mismatch repair. We suggest that the genetic instability of the dam recA730 mutants is provoked by the high level of replication errors induced by the recA730 mutation, generating killing by coincident mismatch repair on the two unmethylated DNA strands. The recA730 mutation increases spontaneous mutagenesis of phage lambda poorly. UV irradiation of recA730 host bacteria increases phage untargeted mutagenesis to the level observed in UV-irradiated recA+ strains. This UV-induced mutator effect in recA730 mutants is not suppressed by a umuC mutation. Therefore UV and the recA730 mutation seem to induce different SOS mutator activities, both generating untargeted mutations.

SOS mutator activity: Unequal mutagenesis on leading and lagging strands

“…These three alleles (when residing on FЈprolac) were shown previously to be particularly responsive to SOS induction, their corresponding mutant frequencies being increased 6-, 18-, and 55-fold, respectively (20). As SOS-induced replication errors are subject to DNA mismatch repair like normal errors (20,36), we again conducted the experiments in a mismatch-repair-defective (mutL) background, permitting a ready analysis of the observed mutant frequencies in terms of replication error frequencies.…”

cAMP-dependent SOS induction and mutagenesis in resting bacterial populations.

“…13-Galactosidase units were calculated according to Miller (20). Detection (24). Cells were plated on either 869 containing rifampicin (100 ,ug/ml) or 869.…”

A Polymerase Tautomeric Model for Radiation Induced Bystander Effects: A Model for Untargeted Base Substitution Mutagenesis during Error Prone and SOS Replication of Double Stranded DNA Containing Thymine and Adenine in Rare Tautomeric Forms

“…It is assumed that DNA polymerases insert non-complementary bases opposite the damages [19,22]. As part of the polymerase model mechanisms of the targeted [26,27], and untargeted [19,20,[28][29][30][31][32][33] base substitution mutations have been proposed. The mutations that result from incorrect bases are often targeted; that is, they occur at the same position as the photoproduct [26,27].…”

“…The mutations that result from incorrect bases are often targeted; that is, they occur at the same position as the photoproduct [26,27]. Sometimes mutations are formed in the vicinity of damage, a process that is termed untargeted mutagenesis [19,20,[28][29][30][31][32][33].Untargeted mutations currently called mutations appearing on so-called "undamaged" DNA sites [19,20,[28][29][30][31][32][33][34][35][36]. Untargeted mutagenesis requires the same proteins that are required for translesion synthesis [34].…”

“…Untargeted mutagenesis is characterized by a high rate of education of transversions [19,34,36]. It is assumed that untargeted base substitution mutations formed on undamaged DNA sites in result in random errors of DNA polymerases [19,20,[28][29][30][31][32][37][38][39].…”