recX, a new SOS gene that is co-transcribed with the recA gene in Escherichia coli.

recX is a small open reading frame located downstream of recA that is conserved in many bacteria. In Escherichia coli, the recX gene (also named oraA) is a 501 bp open reading frame that encodes a predicted basic protein. Transcriptional analysis by Northern blots showed that in E. coli the recX gene is SOS-regulated. Primer extension data and transcriptional fusions indicate that recX transcription is down regulated with respect to recA by an intrinsic transcription terminator that is located between the recA and recX coding sequences. Despite the presence of this terminator, a recA-recX message resulting from transcriptional readthrough is detected at a level of 5-10% of the recA message. In addition, transcriptional/translational fusion experiments show that recX expression is further down regulated at the translational level reaching an estimated protein level about 500-fold lower than RecA. Strains in which the recX gene was disrupted were constructed by insertion of an antibiotic resistance cassette. The survival after UV irradiation, the spontaneous and UV-induced mutation rates were not significantly different in these recX strains compared to the corresponding wild type strain. Overexpression of RecA was shown to be lethal in a recX deletion strain in Pseudomonas aeruginosa [J. Bacteriol. 175 (1993) 2451], Mycobacterium tuberculosis [Mol. Microbiol. 30 (1998) 525] and Streptomyces lividans [J. Bacteriol. 182 (2000) 4005] suggesting that the recX gene may act as a regulator of recA. In contrast in E. coli, in a recX deletion strain, RecA overexpression is neither toxic nor is the expression of the recA(+) gene affected in a recX deletion strain at the basal level or after UV induction.