Induction of the Escherichia coli SOS system increases the ability of the cell to perform DNA repair and mutagenesis. Products of the recA and umuD,C genes are required for mutagenesis induced by radiation and many chemicals. Transcription of the SOS genes including recA and umuD,C is repressed by a repressor, LexA protein, and is derepressed by the proteolytic cleavage of LexA facilitated by RecA protein that had been activated by inducing signals produced in the cell by agents that damage DNA. An activated form of RecA protein, RecA, seems to have roles in SOS mutagenesis other than its known role as an antirepressor. Derepression of the genes involved in SOS mutagenesis such as recA and umuD,C in defective chromosomal lexA(Def) mutants does not increase the ability of the cell to perform mutagenesis. Activation of RecA protein is essential to this ability. RecA facilitates the proteolytic cleavage of several repressors such as lambda, P22, and 434 phage repressors and LexA, and UmuD protein contains a sequence homologous to the regions surrounding the cleavage sites of these repressors; therefore, we examined the possibility that UmuD protein is cleaved by RecA. We found evidence that the intact UmuD protein was cleaved after mutagenic treatment and that the cleavage was dependent on RecA. The results suggested that UmuD protein may be proteolytically processed by RecA, and that processed UmuD may be the active form of the protein participating in mutagenesis.
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