Mutagenesis and growth delay induced in Escherichia coli by near-ultraviolet radiations

The literature relating to genetic changes induced in Escherichia coli by near-ultraviolet radiations is reviewed and summarized: i) these radiations are much less mutagenic than would be expected from the known level of DNA damage, ii) pre-illumination with near-UV light antagonizes the mutagenic effect of UV (254 nm) light. In agreement with these findings, the SOS functions are not induced by near-UV radiations. Furthermore prior exposure of cells to near-UV light inhibits the subsequent 254 nm induction of the SOS response. Among the several hypothesis considered to explain these observations, one can be clearly favoured. Near-UV light triggers, at sublethal fluences, the growth delay effect. The target molecules, tRNAs, are photocrosslinked and some tRNA species become poor substrates in the acylation reaction. In vivo these tRNA molecules accumulate on the uncharged form, leading to a transient cessation of protein synthesis. The SOS response is inducible and as such requires protein synthesis. We therefore propose that near-ultraviolet radiations have a dual effect: i) they induce, mostly indirectly, DNA lesions which are potentially able to trigger the SOS response, ii) they prevent the expression of the SOS functions through the transient inhibition of protein synthesis (growth delay).