Isolates of enterohaemorrhagic serotype 0157:H7 do not exhibit β-glucuronidase (GUD) activity; however, they carry nucleotide sequences for the gene that encodes the GUD enzyme. Polymerase chain reaction analysis using -specific primers confirmed that a genetic region homologous in size to the structural gene, including the regulatory region, was present in 0157:H7 isolates. DNA sequencing analysis of the regulatory region and the 5′ terminus of the gene of 0157:H7 showed a base substitution in the putative −10 promotor region and at 93 bases downstream from the initiation codon. Neither base alteration, however, appeared to affect allele gene expression in the 0157:H7 isolates. Immunoblotting of cell extracts with an anti-GUD antibody showed that 0157:H7 isolates produced an antibody-reactive protein that was homologous in size to GUD, but no GUD activity was observed in cell-free extracts of these isolates. These results suggest that the antibody-reactive protein produced by 0157:H7 may be an inactive GUD enzyme. Sequencing of the structural gene in 0157:H7 showed the presence of 18 additional nucleotide base substitutions, but only six altered the amino acid sequence. Also, there were two frame shift mutations, 18 bases apart, that altered the sequence of six consecutive amino acids. These genetic alterations in the structural gene of 0157:H7 may account for the absence of GUD activity in this serotype.


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