Abstract
Coordination of gene expression in the lac operon was compared in Escherichia coli and Salmonella typhimurium as an approach to detecting possible differences in protein synthesis or membrane structure between organisms. Either a wild-type F' lac pro(AB) episome or the same episome with a polar mutation in one of the lac genes was introduced into pro(-) derivatives of the two strains of bacteria. Activity assays showed that the beta-galactosidase levels were only slightly lower in the S. typhimurium cells than in E. coli cells, whereas the transacetylase levels were significantly higher in S. typhimurium for all of the lac markers tested. Galactoside transport activities were always comparable in the two strains of bacteria; this latter result indicates that the cell envelopes of E. coli and S. typhimurium do not differ sufficiently to affect the membrane-associated lac transport system. It was found, however, that the specific transport activity is very sensitive to culture age in both bacteria, and decreases rapidly in cultures past the mid-exponential phase of growth.
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