ROLE OF MISMATCH REPAIR IN THE ESCHERICHIA-COLI UVM RESPONSE

Mutagenesis at 3,N-4-ethenocytosine (epsilon C), a nonpairing mutageni c lesion, is significantly enhanced in Escherichia coli cells pretreat ed with UV, alkylating agents, or H2O2. This effect, termed UVM (for U V modulation of mutagenesis), is distinct from known DNA damage-induci ble responses, such as the SOS response, the adaptive response to alky lating agents, or the oxyR-mediated response to oxidative agents. Here , we have addressed the hypothesis that UVM results from transient dep letion of a mismatch repair activity that normally acts to reduce muta genesis. To test whether the loss of mismatch repair activities result s in the predicted constitutive UVM phenotype, E. coli cells defective for methyl-directed mismatch repair, for very-short-patch repair, or for the N-glycosylase activities MutY and MutM were treated with the U VM-inducing agent 1-methyl-3-nitro-1-nitrosoguanidine, with subsequent transfection of M13 viral single-stranded DNA bearing a site-specific epsilon C lesion. Survival of the M13 DNA was measured as transfectio n efficiency, and mutation fixation at the lesion was characterized by multiplex sequencing technology. The results showed normal UVR;I indu ction patterns in all the repair-defective strains tested. In addition , normal UVM induction was observed in cells overexpressing MutH, MutL , or MutS. All strains displayed UVM reactivation, the term used to de scribe the increased survival of epsilon C-containing DNA in UVM-induc ed cells. Taken together, these results indicate that the UVM response is independent of known mismatch repair systems in E. coli and may th us represent a previously unrecognized misrepair or misreplication pat hway.