INVOLVEMENT OF RECA AND RECF IN THE INDUCED PRECISE EXCISION OF TN10 IN ESCHERICHIA-COLI

It has been shown that the increased frequency of precise excision of Tn10 observed after UV or mitomycin C (MMC) treatment or with uvrD- mu tants is recA-dependent. Previous work has also shown that expression of SOS genes is required for W-or MMC-induced Tn10 precise excision. I n order to determine if the increased excision of Tn10 in uvrD- mutant s requires only expression of recA, or expression of other SOS genes, or both, we studied the precise excision of Tn10 in lexA3 (Ind-, SOS n on-inducible) and lexA3 recAo98 (operator constitutive recA) mutants. The results of these experiments indicate that the induced excision of Tn10 in the uvrD- null mutant depends on the expression of recA rathe r than on any of the other genes repressed by LexA. The effect of a nu ll recF mutation on the excision of Tn10 in a uvrD- mutant was also in vestigated and found to abolish the increased frequencies of this proc ess. Similarly, the recF mutation was found to decrease markedly the i ncreased precise excision of Tn10 induced by MMC in a uvrD+ isogenic s train. These observations indicate that recA and recF are involved in the increased frequencies of Tn10 excision exhibited by uvrD- mutants or after MMC treatment. It remains to be determined whether these two genes participate in these two induction processes in the same biochem ical pathways. (C) 1997 Elsevier Science B.V.