SUMMARY: Strains of isolated from the blood of patients with typhoid fever invariably express a capsular polysaccharide, termed the Vi antigen. Vi antigen expression is controlled by two separate chromosomal loci, and . The locus is commonly found in enteric bacteria. In contrast, the locus appears to be specific to Vi-expressing strains of and . Here the cloning, expression and analysis of determinants from Ty2 is described. Whole-cell DNA from strain Ty2 was size-fractionated and cloned into the pLA2917 cosmid vector. A recombinant cosmid, pVT1, conferring a Vi-positive phenotype upon and upon the Vi-non-expressing strain Ty21a of , was characterized and used for further studies. Transposon Tn5 insertion mutagenesis demonstrated that the Vi-antigen-encoding region on pVT1 consisted of a 15 kb fragment. A subclone, designated pVT3, which contained an 18 kb insert, was sufficient to confer Vi antigen expression upon and Ty21a. Results of recombination experiments indicated that this DNA sequence was the locus of Ty2. In SE5000 maxicells, the determinants encoded at least eight polypeptides, with molecular masses of 80, 65, 59, 48, 44, 39, 35 and 28 kDa. Functional characterization of mutations in Ty2 suggested that the 80 and 65 kDa proteins were required for cell-surface localization of the Vi antigen.


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