Characterization of Xanthomonas oryzae pv. oryzae recX, a gene that is required for high-level expression of recA.

Analysis of the nucleotide sequence downstream from the Xanthomonas oryzae pv. oryzae recA gene reveals two orfs designated orfX and recX. The former has the potential to code for a 5.6 kDa protein of unknown function while the latter encodes for a putative 14.6 kDa protein with homology to RecX from various bacteria. Northern blot analysis and RT-PCR results show that recA-orfX-recX are co-regulated and arranged in an operon. A recX mutant was constructed. The mutant has no obvious growth defects or stress response defects, except that it cannot support high-level expression of recA from an expression vector. Introduction of the plasmid containing recA into the recX mutant resulted in reduced transformation efficiency and all transformants tested had mutations with reduced RecA levels. Moreover, the recX mutant has reduced basal levels of RecA. This has not been observed in other bacteria. When inactivated recX was complemented in trans, both changes were reversed. recX mutation has no effect on the regulation of the recA promoter, suggesting that its effect on the RecA level could be post-transcriptional.