The genes responsible for O-antigen synthesis of Vibrio cholerae O139 are closely related to those of Vibrio cholerae O22

Abstract

Several studies have shown that the emergence of the O139 serogroup of Vibrio cholerae is a result of horizontal gene transfer of a fragment of DNA from a serogroup other than O1 into the region responsible for O-antigen biosynthesis of the seventh pandemic V. cholerae O1 biotype El Tor strain. In this study, we show that the gene cluster responsible for O-antigen biosynthesis of the O139 serogroup of V. cholerae is closely related to those of O22. When DNA fragments derived from O139 O-antigen biosynthesis gene region were used as probes, the entire O139 O-antigen biosynthesis gene region could be divided into five classes, designated as I-V based on the reactivity pattern of the probes against reference strains of V. cholerae representing serogroups O1-O193. Class IV was specific to O139 serogroup, while classes I-III and class V were homologous to varying extents to some of the non-O1, non-O139 serogroups. Interestingly, the regions other than class IV were also conserved in the O22 serogroup. Long and accurate PCR was employed to determine if a simple deletion or substitution was involved to account for the difference in class IV between O139 and O22. A product of approx. 15 kb was amplified when O139 DNA was used as the template, while a product of approx. 12.5 kb was amplified when O22 DNA was used as the template, indicating that substitution but not deletion could account for the difference in the region between O22 and O139 serogroups. In order to precisely compare between the genes responsible for O-antigen biosynthesis of O139 and O22, the region responsible for O-antigen biosynthesis of O22 serogroup was cloned and analyzed. In concurrence with the results of the hybridization test, all regions were well conserved in O22 and O139 serogroups, although wbfA and the five or six genes comprising class IV in O22 and O139 serogroups, respectively, were exceptions. Again the genes in class IV in O22 were confirmed to be specific to O22 among the 155 'O' serogroups of V. cholerae. These data suggest that the gene clusters responsible for O139 O-antigen biosynthesis are most similar to those of O22 and genes within class IV of O139, and O22 defines the unique O antigen of O139 or O22.