Previous studies of doxorubicin‐induced mutations employing F lacl/lacO as an endogenous gene target have focused on properties of large deletions with 3′ endpoints residing in the lacO region of the target gene. This study considers the influence of Lac represser binding on the distribution of these deletions. Results of the DNA sequence level analysis of spontaneous and doxorubicin‐induced i−d and lacO mutations in Escherichia coli uvrB− are reported for mutants isolated under conditions where Lac repression is relieved by isopropyl‐β‐D‐thioga‐lactopyranosid (IPTG; an inducer that prevents represser binding to lacO). The location of deletions isolated from doxorubicin‐treated cultures in the presence and absence of IPTG suggests that doxoru‐bicin preferentially focuses deletion endpoints adjacent to its binding sites in lacO and that the distribution of these deletion endpoints is not modulated by Lac represser binding. In contrast, spontaneous deletion endpoints are preferentially clustered in the loop away from the palindromic sequences under conditions of repression. However, when the Lac repressor/lacO binding complex is dissociated by IPTG, the spontaneous 3′‐deletion endpoints distribute proportionally between the putative stem 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 lacO. This base substitution “hot spot,” which accounted for 17.6% of total doxorubicin‐induced mutants and 16.4% of spontaneous mutants in repressed bacterial cultures, accounted for approximately 1% of total mutations in similar experiments carried out in the presence of IPTG. A large number of mutations at the +6 position are induced only by doxorubicin in the absence of IPTG, however, suggesting that both doxorubicin‐induced and spontaneous mutation at this transition “hot spot” are mediated by Lac represser binding to lacO. © 1995 Wiley‐Liss, Inc.
- DNA sequence
ASJC Scopus subject areas
- Health, Toxicology and Mutagenesis