Near ultraviolet DNA damage induces the SOS responses in Escherichia coli

The influence of the growth delay induced by near u.v. radiation on the SOS response was monitored by comparing the level of sfiA expression by means of a sfiA::lacZ fusion in both a nuvA+ cell and an isogenic nuvA mutant. The mutant lacks 4-thiouridine in its tRNA and does not exhibit the near u.v.-induced growth delay. Although the two strains exhibit similar sfiA induction levels after 254 nm irradiation, their behaviour is different after illumination with near u.v. light, including solar u.v. Inducibility is 10-20 times higher in the nuvA mutant than in the parent strain. Furthermore, pre-illumination with broad band near u.v. light does not affect the 254 nm-induced sfiA response in the mutant but reduces it by a factor of 3-4 in the parent strain. The kinetics of sfiA induction in near u.v.-illuminated nuvA+ cells, whether treated with 254 nm light or not, is unusual and follows the growth curve: only after 50 min is sfiA derepression observed. It can be concluded that (i) near u.v.-induced DNA lesions are able to trigger the SOS response and (ii) the growth delay effect reduces this response, whether triggered by u.v. or near u.v. light. Hence 4-thiouridine in tRNA acts as a built-in antiphotomutagenic 'device' protecting Escherichia coli cells against mutagenesis and the induction of the SOS response by near u.v. light and sunlight.