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Antigens and Immunity |
Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College, Dublin 2, Ireland1
Author for correspondence: Peter Owen. Tel: +353 1 6081188. Fax: +353 1 6799294. e-mail: powen{at}tcd.ie
agn43 encodes a major phase-variable outer-membrane protein, antigen 43 (Ag43), involved in autoaggregation of Escherichia coli cells. The gene is present in single copy on the chromosome of E. coli K-12. In contrast, Southern hybridization and gene inactivation studies demonstrate that control producer strain E. coli ML308-225 possesses duplicate copies of agn43 (agn43A and agn43B). 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 agn43A and agn43B gene sequences revealed 98% identity at the nucleotide and predicted protein levels, with differences in the protein sequence of the surface-expressed 
43 subunit altering the surface probability of one of the predicted epitopes. Analysis of a panel of enteropathogenic E. coli strains by Southern hybridization using agn43-specific gene probes provided strong evidence for the presence of varying numbers of agn43 alleles within clinical isolates. Taken together, the results indicate the presence of a family of distinct Ag43 proteins encoded by multiple chromosomal alleles.
Keywords: antigen 43, Escherichia coli, phase variation, multiple alleles
Abbreviations: Ag43, antigen 43; Dam, deoxyadenosine methylase; EPEC, enteropathogenic E. coli; DIG, digoxigenin
The GenBank accession numbers for the sequences reported in this paper are AF233271–AF233273.
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