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A recA deletion mutant of Mycobacterium smegmatis has been isolated by homologous recombination using a sacB counterselection strategy. Deletion of the recA gene from the chromosome was demonstrated by Southern hybridizations and by polymerase chain reaction (PCR). Western analysis using anti-RecA antibodies confirmed that the RecA protein was not made by the mutant strain. The recA deletion strain exhibited enhanced sensitivity to UV irradiation and failed to undergo homologous recombination. The results obtained from the recombination assays suggest that in wild-type M. smegmatis the majority of colonies arise from single cross-over homologous recombination events with only a very minor contribution from random integrations. The deficiencies in UV survival and recombination were complemented by introduction of the cloned M. smegmatis recA gene. Overexpression of RecA was found to be toxic in the absence of recX, which is found downstream of and co-transcribed with recA and is thus also affected by the deletion of recA. The M. smegmatis recA deletion strain was also complemented by the M. tuberculosis recA gene with or without its intein; most importantly, the frequency of double cross-over homologous recombination events was identical regardless of whether the M. tuberculosis recA gene contained or lacked the intein. Thus, the low frequency of homologous recombination observed in M. tuberculosis is not due to the presence of an intein-coding sequence in its recA gene per se.