Molecular characterization of the proU loci of Salmonella typhimurium and Escherichia coli encoding osmoregulated glycine betaine transport systems.

The proU loci of Salmonella typhimurium and Escherichia coli encode high-affinity glycine betaine transport systems which play an important role in survival under osmotic stress. Transcription of the proU locus is tightly regulated by osmolarity and this regulation appears to be mediated by osmotically induced changes in DNA supercoiling. In order to study the regulatory mechanisms involved we have cloned and characterized the proU locus of S. typhimurium by an in vivo transductional procedure. The locus is shown to consist of at least three genes, designated proVWX, cotranscribed as a single operon. The first gene in the operon encodes a protein sharing considerable sequence identity with ATP-binding proteins from other periplasmic transport systems. Unexpectedly, the highly expressed periplasmic glycine betaine binding protein was found to be encoded by a distal gene, proX, in the operon. The operon has no significant internal promoters but is expressed from a single osmoregulated promoter whose transcription start site has been mapped. The proU promoter of E. coli has also been sequenced and the transcription start site shown to be similar to that of S. typhimurium. Evidence is presented which suggests that, besides de novo glycine betaine uptake, an important function of ProU may be the recapture and recycling of other osmolytes that leak from the cell.