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Genetic analysis of Shigella sonnei form I antigen: identification of a novel IS630 as an essential element for the form I antigen expression.

The form I coding region of Shigella sonnei was cloned and shown to have an operon-like rfb organization. It was found that the 11.0 kb HindIII-XbaI fragment of pHH201 encoding the form I antigen contains 10 contiguous open reading frames (ORF), ORF1 to ORF10. Deletions from either end of pHH201, within ORF1 or ORF10, eliminated form I expression. ORF1 and ORF2 share significant nucleic and amino acids homologies to two ORF's of the Salmonella typhi Vi antigen genes. ORF5 in pHH201 is identical to IS630. pHH2064, derived from pHH201, lacks the IS630 element and can stably express the form I antigen inE. coli HB101. However, pHH2064 is structurally unstable in a S. sonnei form II host. This indicates that the presence of the IS630 gene within the S. sonnei rfb operon may be necessary for the stability of form I expression in S. sonnei. This finding is substantiated by the observation that all virulent S. sonnei isolates examined in this study retained the IS630 element within their rfb operon.