Transcriptional and mutational analyses of the Streptomyces lividans recX gene and its interference with RecA activity

The role of the 20,922-Da RecX protein and its interference with RecA activ ity were analyzed in Streptomyces lividans. The recX gene is located 220 bp downstream of recA. Transcriptional analysis by reverse transcriptase PCR demonstrated that recX and recA constitute an operon. While recA was transc ribed at a basal level even under noninducing conditions, a recA-recX cotra nscript was only detectable after induction of recA following DNA damage. T he recA-recX cotranscript was less abundant than the recA transcript alone. The recX gene was inactivated by gene replacement. The resulting mutant ha d a clearly diminished colony size, but was not impaired in recombination a ctivity, genetic instability, and resistance against UV irradiation, Expres sion of an extra copy of the S. lividans recA gene under control of the thi ostrepton-inducible tipA promoter was lethal to the recX mutant, demonstrat ing that RecX is required to overcome the toxic effects of recA overexpress ion. Since inactivation of the recX gene did not influence transcription of recA, the putative function of the RecX protein might be the downregulatio n of RecA activity by interaction with the RecA protein or filament.