Hypermutagenesis in mutA cells is mediated by mistranslational corruption of polymerase, and is accompanied by replication fork collapse

Elevated mistranslation induces a mutator response termed translational stress-induced mutagenesis (TSM) that is mediated by an unidentified modification of DNA polymerase III. Here we address two questions: (i) does TSM result from direct polymerase corruption, or from an indirect pathway triggered by increased protein turnover? (ii) Why are homologous recombination functions required for the expression of TSM under certain conditions, but not others? We show that replication of bacteriophage T4 in cells expressing the mutA allele of the glyV tRNA gene (Asp -> Gly mistranslation), leads to both increased mutagenesis, and to an altered mutational specificity, results that strongly support mistranslational corruption of DNA polymerase. We also show that expression of mutA, which confers a recA-dependent mutator phenotype, leads to increased lambdoid prophage induction (selectable in vivo expression technology assay), suggesting that replication fork collapse occurs more frequently in mutA cells relative to control cells. No such increase in prophage induction is seen in cells expressing alaV(Glu) tRNA (Glu -> Ala mistranslation), in which the mutator phenotype is recA-independent. We propose that replication fork collapse accompanies episodic hypermutagenic replication cycles in mutA cells, requiring homologous recombination functions for fork recovery, and therefore, for mutation recovery. These findings highlight hitherto under-appreciated links among translation, replication and recombination, and suggest that translational fidelity, which is affected by genetic and environmental signals, is a key modulator of replication fidelity.