encodes a major phase-variable outer-membrane protein, antigen 43 (Ag43), involved in autoaggregation of cells. The gene is present in single copy on the chromosome of K-12. In contrast, Southern hybridization and gene inactivation studies demonstrate that control producer strain ML308-225 possesses duplicate copies of ( and ). Construction and analyses of single and double knockout mutants clearly show that both alleles are capable of expressing antigen in a phase-variable manner, with observed differences in the ON↔OFF switch frequencies appearing to favour expression of Ag43B under conditions of normal laboratory growth. Comparative analysis of and gene sequences revealed 98% identity at the nucleotide and predicted protein levels, with differences in the protein sequence of the surface-expressed α subunit altering the surface probability of one of the predicted epitopes. Analysis of a panel of enteropathogenic strains by Southern hybridization using -specific gene probes provided strong evidence for the presence of varying numbers of alleles within clinical isolates. Taken together, the results indicate the presence of a family of distinct Ag43 proteins encoded by multiple chromosomal alleles.


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