UDP-sugar hydrolase isozymes in Salmonella enterica and Escherichia coli: silent alleles of ushA in related strains of group I Salmonella isolates, and of ushB in wild-type and K12 strains of E. coli, indicate recent and early silencing events, respectively.

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) occurred relatively early following the divergence of Escherichia coli and Salmonella from a common ancestor that was ushA+ ushB+.

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