ON THE INDUCTION OF PROTECTIVE RESPONSES IN SALMONELLA-TYPHIMURIUM STRAIN TA1535 PSK1002 BY UVA (365 NM)/

Exposure to UVA (365 nm) led to growth delay, loss of viability and in hibition of H-3-thymidine incorporation into the cells of Salmonella t yphimurium strain TA1535 containing multiple copies of a plasmid pSK10 02 carrying a umuC'-'lacZ fusion gene. Ultraviolet-A induced umu gene expression, as monitored by the estimation of beta-galactosidase, in a linear fluence-dependent manner. The induction of umu gene expression increased with the increase of postirradiation incubation period of t he cells in the LB-ampicillin (LBA) medium at 37 degrees C and leveled off from 2 h onward. The induction of gene expression depended on con comitant protein synthesis and represented the induction of the SOS re sponse in the particular S. typhimurium cells used. The exposure to lo w fluences (sublethal) of UVA also led to the induction of an adaptive response in the same bacterial cells, which made them resistant to su bsequent challenge by a much higher fluence of the same radiation. The adaptive response, as monitored by the assays of viability and beta-g alactosidase units, increased with the period of exposure to sublethal fluences of UVA, attained a maximum at the UVA exposure of 4.5 kJ/m(2 ) (15 min) and thereafter gradually decreased with further increase of UVA exposure period. Modulation studies involving D2O, LBA growth med ium, different scavengers of free radicals and quenchers of activated oxygen species indicated the involvement of both hydroxyl free radical s and singlet oxygen in the UVA-induced umu gene expression.