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Genetic Evidence for the Requirement of RecA Loading Activity in SOS Induction after UV Irradiation in Escherichia coli

The SOS response in Escherichia coli results in the coordinately expression of more than 40 genes which occurs when cells are treated with DNA-damaging agents. This response is dependent on RecA (coprotease), LexA (repressor), and the presence of single-stranded DNA (ssDNA). A prerequisite for SOS induction is the formation of a RecA-ssDNA filament.Depending on the DNA substrate, the RecA-ssDNA filament is produced by either RecBCD, RecFOR, or a hybrid recombination mechanism with specific enzyme activieties, including helicase, exonuclease, and RecA loading. In this study we examined the role of RecA loading activities in SOS induction after UV irradiation. We performed a genetic analysis of SOS induction in strains with a mutation which eliminates RecA loading activity in the RecBCD enzyme (recB1080 allele). We found that RecA loading activity is essential for SOS induction. In the recB1080 mutant RecQ helicase is not important, whereas RecJ nuclease slightly decreases SOS induction after UV irradiation. In addition, we found that the recB1080 mutant exhibit constitutive expression of the SOS regulon. Surprisingly, this constitutive SOS expression was dependent on the RecJ protein but not on RecFOR, implaying that there is a different mechanism of RecA loading for constitutive SOS expression.

Escherichia coli. RecBCD enzyme; RecA loading activity; SOS induction. UV irradiation

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