IMPROVED ALLELIC EXCHANGE VECTORS AND THEIR USE TO ANALYZE 987P FIMBRIA GENE-EXPRESSION

A series of vectors has been developed to provide improved positive an d negative selection for allelic exchange. Based on homologous regions of DNA ranging in size from less than 200 bp to over 1 kb, we have su ccessfully used these new plasmids to introduce or remove markers in c hromosomal or plasmid DNA. Wild type fimbria genes were replaced both in Salmonella enteritidis (sefA, agfA and fimC) and Escherichia coli ( fasA and fasH). Regulation of 987P fimbriation could be identified aft er replacement of fasA and fasH with allelic reporter fusions. The exp ression of fasA but not fasH is dependent upon the osmolarity of the g rowth medium in an HNS-dependent manner, but unlike some other fimbria l systems expression is not dependent on the exogenous iron concentrat ion. (C) 1998 Elsevier Science B.V.