UVM, AN ULTRAVIOLET-INDUCIBLE RECA-INDEPENDENT MUTAGENIC PHENOMENON IN ESCHERICHIA-COLI

Most mutagenic DNA lesions are noninstructive in the sense that templa te instruction is either missing or inaccessible during DNA replicatio n, leading to replication arrest. According to the SOS hypothesis, arr ested replication induces the expression of SOS factors that force rep lication past stalled sites at the cost of mutagenesis. We have recent ly shown that prior UV irradiation of Delta recA cells, in which the S OS pathway does not function, enhances mutagenesis at an ethenocytosin e residue borne on a circular gapped duplex DNA vector, indicating the existence of an SOS-independent inducible mutagenic phenomenon termed UVM (UV modulation of mutagenesis). In the previous experiments, muta tion fixation was expected to occur during gap-filling DNA synthesis. To test whether UVM is observable during normal replication by DNA pol ymerase III, we have examined mutagenesis at an epsilon C residue born e on M13 single-stranded DNA. By analyzing mutation frequency and spec ificity using a multiplex sequence assay, we now show that UVM is obse rvable in UV-irradiated recA(+), and in Delta recA cells. These data i ndicate that UV irradiation induces a previously unrecognized mutageni c mechanism in Escherichia coli, and that this mechanism is manifested during gap-filling DNA synthesis as well as during normal DNA replica tion.