Abstract
recA1, recA13 and recA56 are considered null alleles of the Escherichia coli recA gene because they were shown to have essentially no activity in vivo. In this study, we used strains harboring the recA null alleles and their recA-proficient congenic counterpart to assess the lethal and the mutagenic effects elicited by near-UV(308 nm) coherent radiation generated by a XeCl excimer laser. We compared these effects with those produced by a conventional far-UV(254 nm) germicidal lamp. Compared to the germicidal lamp, the excimer laser was able to better discriminate the different recA-defective strains on the basis of their UV-radiation sensitivity, which was progressively higher in the strains with the alleles in the order recA1, recA56 and recA13. This finding was consistent with previous data on residual biochemical activities of the respective mutated RecA proteins in vitro. The discrepancy between the results obtained with the lamp and laser irradiation suggested that the biological response to the two radiations involves distinct mechanisms. This hypothesis was supported by the evidence that exposure to near-UV(308 nm) radiation induced mutagenesis in recA-defective strains at an extent considerably greater than in recA-proficient strains. In contrast, far-UV(254 nm)-radiation-induced mutagenesis was reported to be largely dependent on a functional recA allele.
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