ALKYLATING-AGENTS INDUCE UVM, A RECA-INDEPENDENT INDUCIBLE MUTAGENIC PHENOMENON IN ESCHERICHIA-COLI

Noninstructive DNA damage in Escherichia coli induces SOS functions hy pothesized to be required for mutagenesis and translesion DNA synthesi s at noncoding DNA lesions. We have recently demonstrated that in E. c oli cells incapable of SOS induction, prior UV-irradiation nevertheles s strongly enhances mutagenesis at a noninstructive lesion borne on M1 3 DNA. Here, we address the question whether this effect, named UVM fo r UV modulation of mutagenesis, can be induced by other DNA damaging a gents. Exponentially growing Delta recA cells were pretreated with alk ylating agents before transfection with M13 single-stranded DNA bearin g a site-specific ethenocytosine residue. Effect of cell pretreatment on survival of the transfected DNA was determined as transfection effi ciency. Mutagenesis at the ethenocytosine site in pretreated or untrea ted cells was analyzed by multiplex DNA sequencing, a phenotype-indepe ndent technology. Our data show that 1-methyl-3-nitro-1-nitrosoguanidi ne, N-nitroso-N-methylurea and dimethylsulfate, but not methyl iodide, are potent inducers of UVM. Because alkylating agents induce the adap tive response to defend against DNA alkylation, we asked if the genes constituting the adaptive response are required for UVM. Our data show that MNNG induction of UVM is independent of ada, alkA and alkB genes and define UVM as an inducible mutagenic phenomenon distinct from the E, coli adaptive and SOS responses.