• Silent gene;
  • UDP-sugar hydrolase;
  • 5′-Nucleotidase;
  • Isozymes;
  • Salmonella;
  • Escherichia coli

Abstract Escherichia coli contains a single periplasmic UDP-glucose hydrolase (5′-nucleotidase) encoded by ushA. Salmonella enterica, serotype Typhimurium, also contains a single UDP-glucose hydrolase but, in contrast to E. coli, it is membrane-bound and is encoded by the non-homologous ushB gene; Salmonella enterica (Typhimurium) also contains a silent allele of the ushA gene (ushA0). In this report, we show that nearly all natural isolates of Salmonella contain both UDP-sugar hydrolases, i.e. they are UshA+ UshB+. The only exceptions are all from sub-group I (S. gallinarum, S. pullorum, and most Typhimurium strains), are UshA UshB+, and several have been shown to contain an ushA0 allele. These data, together with the fact that these latter strains are closely related genetically, strongly suggests a recent silencing mutation(s). We also report the presence in E. coli K-12, and in natural isolates of E. coli, of a DNA sequence which is homologous to the ushB gene of Salmonella; since E. coli does not contain UshB activity, we tentatively refer to this sequence as ushB0. Since all E. coli strains investigated are UshB, we conclude that the silencing mutation(s) occured relatively eary following the divergence of Escherichia coli and Salmonella from a common ancestor that was ushA+ ushB+.