Effect of point mutations in the lac promoter on transient and severe catabolite repression of the lac operon of Escherichia coli

摘要

p1. Experiments were devised to show whether the point mutations L8 and L29 in the ilac/i promoter alleviate transient repression. 2. Several recombinants were picked from matings between a single Fsup?/supip/isup+/supstrain and Hfr strains carrying mutations L8 and L29. All of the 19 ip/isup?/suprecombinants tested proved to suffer no transient repression, whereas all of the eight ip/isup+/suprecombinants tested suffered prolonged transient repression. 3. A diploid strain was constructed in which more than 90% of the thiogalactoside transacetylase is synthesized from the episome with a wild-type ilac/i promoter, whereas 100% of the β-galactosidase is synthesized from the chromosome with a promoter carrying mutation L8. In this diploid the synthesis of thiogalactoside transacetylase suffered transient repression but the synthesis of β-galactosidase did not. 4. Exactly similar results were obtained with a diploid strain in which the chromosomal promoter carried mutation L29. 5. The same diploid strains were used in experiments to show whether mutations L8 and L29 alleviate the severe catabolite repression caused by growth in glucose plus gluconate. In both strains glucose+gluconate repressed the synthesis of β-galactosidase much less than the synthesis of thiogalactoside transacetylase. 6. These and previously reported results can be explained by assuming (ia/i) that both mutations L8 and L29 render the ilac/i promoter partially, but not completely, insensitive to catabolite repression, and (ib/i) that transient repression is an exceptionally severe form of catabolite repression./p