Nucleotide Sequences of the arb Genes, Which Control r-Glucoside Utilization in Erwinia chrysanthemi: Comparison with the Escherichia coli bgl Operon and Evidence for a New 3-Glycohydrolase Family Including Enzymes from

The phytopathogenic bacterium Erwinia chrysanthemi, unlike other members of the family Enterobac-teriaceae, is able to metabolize the,B-glucosides, arbutin, and salicin. A previous genetic analysis of the E. chrysanthemi arb genes, which mediate,-glucoside metabolism, suggested that they were homologous to the Escherichia coli K-12 bgl genes. We have now determined the nucleotide sequence of a 5,065-bp DNA fragment containing three genes, arbG, arbF, and arbB. Deletion analysis, expression in minicell systems, and comparison with sequences of other proteins suggest that arbF and arbB encode a,3-glucoside-specific phosphotransferase system-dependent permease and a phospho-o-glucosidase, respectively. The ArbF amino acid sequence shares 55 % identity with that of the E. coli BglF permease and contains most residues thought to be important for a phosphotransferase. One change, however, was noted, since BglF Arg-625, presumably involved in phosphoryl transfer, was replaced by a Cys residue in ArbF. An analysis of the ArbB sequence led to the definition of a protein family which contained enzymes classified as phospho-,-glucosidases, phospho-I8-galactosidases, I-glucosidases, and j8-galactosidases and originating from gram-positive and gram-negative bacteria, archebacteria, and mammals, including humans. An analysis of this family allowed us (i) to speculate on the ways that these enzymes evolved, (ii) to identify a glutamate residue likely to be a key amino acid in the catalytic activity of each protein, and (iii) to predict that domain II of the human lactate-phlorizin hydrolase