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 ESCHERICHIA-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 (ushA(O)). 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 ushA(O) 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 ushB(O). 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(+).