Expression and regulation of lactose genes carried by plasmids.

A number of plasmids carrying the lactose character have been studied. All of the plasmids examined so far code for proteins essential for lactose utilization, i.e., beta-galactosidase and galactoside permease. None of them carries enzymatically or immunologically detectable thiogalactoside transacetylase. The expression of the two enzymes is both negatively and positively controlled: they are inducible by different galactosides and are sensitive to catabolite repression. Since the plasmid-coded lactose systems have many features in common with the Escherichia coli lactose operon, it is suggested that the plasmids could have acquired the lactose genes from an E. coli chromosome.