Summary: Interspecific complementation of an mutant with a genomic library was used to identify a recombinant plasmid encoding the gene. Recombinant strains harbouring the gene were isolated by replica-plating bacterial colonies on medium containing methyl methanesulphonate (MMS). MMS-resistant clones were identified as encoding the analogue by their ability to protect HB101 from UV exposure and promote homologous recombination. Subcloning of selected restriction fragments and Tn5 mutagenesis localized the gene to a 1·7 kb II-RI fragment. Analysis of minicell preparations harbouring a 1·9 kb RI fragment containing the coding segment revealed a single 37·5 kDa protein. Insertional inactivation of the cloned gene by Tn5 resulted in the disappearance of the 37·5 kDa protein, concomitant with the loss of RecA function. The gene product did not promote induction of a δ lysogen; instead, the presence of the heterologous gene caused a significant reduction in spontaneous and mitomycin-C-induced prophage induction in and backgrounds. Despite the lack of significant genetic homology between the gene and the counterpart, the RecA protein was nearly identical to that of in molecular mass, and the two proteins showed antigenic cross-reactivity. Western blot analysis of UV-treated revealed a significant increase in RecA antigen in irradiated versus control cells, suggesting that the gene is regulated in a manner similar to that of .


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