Journal of General Microbiology (1 986), 132, 2677-2684. Printed in Great Britain 2677 Effect of Alkylating Agents on the Expression of Inducible Genes of Escherichia coli By J.-ALBERT VERICAT,* RICARDO GUERRERO AND JORDI BARBE Department of Microbiology and Institute for Fundamental Biology, Autonomous University of Barcelona, Bellaterra (Barcelona), Spain (Receiued I1 March 1986; revised 23 May 1986) Increasing doses of alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine, diethyl sulphate and ethylmethane sulphonate cause an inhibition of the expression of the recA and sjA genes of wild-type Escherichia coli. This behaviour was not observed in a lexA.56 mutant which has a defective LexA repressor that is unable to bind to the SOS operator. Furthermore, an ada-1 mutant showed the same behaviour as the wild-type strain indicating that the adaptive proteins are not responsible for the inhibition of recA and sjA at high doses of alkylating agents. These results suggest that the inhibitory effect of these alkylating agents may be found in the interaction between the LexA repressor and the control regions of sfiA and recA. On the other hand, high doses of either UV light or mitomycin C produced only a slight decrease in the induction of recA and sjA, whereas bleomycin had no effect. The fact that a repressor structurally related to LexA repressor, such as Lac1 protein, showed the same behaviour as the LexA repressor when a Lac+ strain was treated with alkylating agents, suggests that these compounds can modify the binding abilities of repressors to DNA, producing a limited or even abolished release of repressors, and so decreasing the expression of inducible genes. INTRODUCTION When normal DNA replication is inhibited by different physical (UV or y radiation), chemical (alkylnitrosoguanidines, mitomycin C, bleomycin), or physiological (thymine starvation, dnaB mutants growing at 42 "C) treatments, a set of cellular responses are induced that are known collectively as the SOS functions (Witkin, 1974; Walker, 1984). These functions include inhibition of cell division, cessation of respiration, induction and reactivation of prophages, error-prone repair activity and increased synthesis of RecA protein. This pleiotropic response depends on the red, lexA and ssb genes (Little & Mount, 1982). It has been shown that LexA protein is the repressor of the SOS genes. The basal level of RecA protein is activated to a protease by an inducing signal after DNA damage. This signal is increased by the activity of the RecBC enzyme exonuclease V (Oishi et al., 1979; Barbk et al., 1985). Once activated, RecA protease can cleave LexA repressor and other SOS-related repressors, resulting in the expression of the SOS genes (Roberts et al., 1978; Little et al., 1980; Kenyon & Walker, 1981 ; Schendel et al., 1982). Previous studies have shown that the induction of several SOS functions is not an all-or-none process but a discriminated one (Guerrero & Barb&, 1982). This discriminated expression of the SOS system is specially clear in the case of alkylating agents such as N-methyl-N'-nitro-N- nitrosoguanidine (MNNG), diethyl sulphate (DES) and ethylmethane sulphonate (EMS) (Barbk et al., 1983). Thus, UV irradiation or bleomycin treatment induce cessation of respiration, filamentous cell growth and prophage induction. In contrast, the three alkylating Abbreviations: MNNG, N-methyl-N:-nitro-N-nitrosoguanidine; DES, diethyl sulphate; EMS, ethylmethane sulphonate ; IPTG, isopropyl b-Dthiogalactopyranoside. 0001-3317 0 1986 SGM