Genetic evidence for modulation of the activator by two regulatory proteins involved in the exogenous induction of phosphoglycerate transport in Salmonella typhimurium.

Previous work from this laboratory has identified in a fragment of DNA, cloned from Salmonella typhimurium, two genes involved in the exogenous induction of phosphoglycerate transport. These two genes, the transporter gene, pgtP, and the activator gene, pgtA, are closely linked physically; they are only 3.4 kilobases apart. In the accompanying paper, we describe the determination of the nucleotide sequence of this 3.4-kilobase DNA segment and show that this segment contains two genes, pgtB and pgtC, encoding two polypeptides of 593 and 397 amino acid residues, respectively. This paper presents an analysis of the effects of insertions and deletions in pgtBC on the expression of pgtP gene and on the expression of lacZ fused to the pgtP gene. The results indicate that both pgtBC genes are necessary for expression of the pgtP gene. Strikingly, deletion of both genes resulted in a constitutive phenotype, suggesting that PgtB and PgtC polypeptides modulate PgtA activity. The expression of the pgtP gene appears to be regulated by the pgtA gene product, which acts as an activator. A model of induction is proposed in which the central feature is the interaction of the three regulatory proteins in the membrane such that the activity of the activator (PgtA) is subject to modulation by the binding of an inducer.