EFFECT OF ISOPROPYL-BETA-D-THIOGALACTOPYRANOSID INDUCTION OF THE LAC OPERON ON THE SPECIFICITY OF SPONTANEOUS AND DOXORUBICIN-INDUCED MUTATIONS IN ESCHERICHIA-COLI

Previous studies of doxorubicin-induced mutations employing F' lacl/la cO as an endogenous gene target have focused on properties of large de letions with 3' endpoints residing in the lacO region of the target ge ne. This study considers the influence of Lac repressor binding on the distribution of these deletions. Results of the DNA sequence level an alysis of spontaneous and doxorubicin-induced i(-d) and lacO mutations in Escherichia coli uvrB(-) are reported for mutants isolated under c onditions where Lac repression is relieved by isopropyl-beta-D-thiogal actopyranosid (IPTG; on inducer that prevents repressor binding to lac O). The location of deletions isolated from doxorubicin-treated cultur es in the presence and absence of IPTG suggests that doxorubicin prefe rentially focuses deletion endpoints adjacent to its binding sites in lacO and that the distribution of these deletion endpoints is not modu lated by Lac repressor binding. in contrast, spontaneous deletion endp oints are preferentially clustered in the loop away from the palindrom ic sequences under conditions of repression. However, when the Loc rep ressor/lacO binding complex is dissociated by IPTG, the spontaneous 3' -deletion endpoints distribute proportionally between the putative ste m and loop of the lacO palindrome. The single most striking effect of IPTG induction of the Lac operon was elimination of a ''hot spot'' for T:A-->C:G transitions at position +6 in locO. This base substitution ''hot spot,'' which accounted for 17.6% of total doxorubicin-induced m utants and 16.4% of spontaneous mutants in repressed bacteriol culture s, accounted for approximately 1% of total mutations in similar experi ments carried out in the presence of IPTG. A large number of mutations at the +6 position ore induced only by doxorubicin in the absence of IPTG, however, suggesting that both doxorubicin-induced and spontaneou s mutation at this transition ''hot spot'' are mediated by Loc repress or binding to lacO. (C) 1995 Wiley-Liss, Inc.