[Expression regulation of purine biosynthetic genes in Salmonella typhimurium. VI. Isolation and characterization of super-repressor mutants].

Starting from strain of Salmonella typhimurium purD::lac, 86 exponential cultures were mutagenized with NTG and white or light blue clones on E + ado + Xgal plate were selected as candidates of purRs mutant. Total 66 independent candidate strains were obtained. By assaying their beta-galactosidase activity under the repressed and derepressed conditions, determining their frequency of revertional mutation, Conducting transductional analysis of mutational site and dorminance test, 11 candidates strains were proved to be super-repressor mutants. These mutants are useful for studying the expression of purine biosynthetic gene and relationship between protein structure and function in general.