Photosensitized UVA light induction of the SOS response in Escherichia coli

Several of the factors controlling the extent of the ultraviolet light (and particularly of UVA 320 nm less than lambda less than 380 nm) induced SOS response in E. coli have been studied using a sfiA::lacZ fusion. The decreased 254 nm induced sfiA expression level triggered by a UVA-induced growth delay (Caldeira de Araujo A. & Favre A. (1986) Embo J., 5, 175-179), is closely mimicked by a transient chloramphenicol protein synthesis inhibition. In a nuvA mutant strain (lacking the growth delay effect), UVA light triggers a 30-40% lower SOS response at temperatures higher than 20 degrees C when illumination is performed under anaerobic conditions: endogenous oxygen-mediated photosensitized reactions appear to contribute to the SOS response. In contrast to the temperature independence of the sfiA induction levels obtained after 254 nm irradiation, the UVA induced response is 30-60% lower when the temperature (T) increases from a value lower than 10 degrees C to a value higher than 20 degrees C. This indicates that detoxifying enzymes play a role at T greater than 20 degrees C. Also the in vitro photooxydation of NADH to give NAD+ is described and its possible role in endogenous photosensibilizations discussed. To explain the contrasted mutagenic efficiencies of UVA light treatment when applied to cells in buffer at high fluences, and to growing cells at low fluence rates, we propose that intrinsically the UVA-induced DNA damages are able to trigger the SOS response (cyclobutyl pyrimidine dimers and some O2-dependent lesions) but also constitute premutagenic sites (some lesions leading to alkali-labile DNA breaks).(ABSTRACT TRUNCATED AT 250 WORDS)