Evidence that an additional mutation is required to tolerate insertional inactivation of the Streptomyces lividans recA gene

In contrast to recA of other bacteria, the recA gene of Streptomyces livida ns has been described as indispensable for viability (G. Muth, D. Frese, A. Kleber, and W. Wohlleben, Mel. Gen. Genet. 255:420-428, 1997.). Therefore, a closer analysis of this gene was performed to detect possible unique fea tures distinguishing the Streptomyces RecA protein from the well-characteri zed Escherichia coli RecA protein. The S. lividans recA gene restored UV re sistance and recombination activity of an E. coli recA mutant. Also, transc riptional regulation was similar to that off. coli recA. Gel retardation ex periments showed that S. lividans recA is also under control of the Strepto myces SOS repressor LexA. The S. lividans recA gene could be replaced only by simultaneously expressing a plasmid encoded recA copy. Surprisingly, the recA expression plasmid could subsequently be eliminated using an incompat ible plasmid without the loss of viability. Besides being UV sensitive and recombination deficient, all the mutants were blocked in sporulation. Genet ic complementation restored UV resistance and recombination activity but di d not affect the sporulation defect. This indicated that all the recA mutan ts had suffered from an additional mutation, which might allow toleration o f a recA deficiency.