GENETIC REARRANGEMENTS IN THE RFB REGIONS OF VIBRIO-CHOLERAE O1 AND O139

The recent emergence of a pathogenic new non-O1 serotype (O139) of Vib rio cholerae has led to numerous studies in an attempt to identify the origins of this nov strain. Our studies indicate that O139 strains ha ve clear differences in the surface polysaccharides when compared with O1 strains: the lipopolysaccharide can be described as semi-rough. So uthern hybridization with the O1 rfb region demonstrates that O139 str ains no longer contain any of the rfb genes required for the synthesis of the O1 O-antigen or its modification and also lack at least 6 kb o f additional contiguous DNA. However, O139 strains have retained rfaD and have a single open reading frame closely related to three small op en reading frames of the O1 rfb region. This region is closely related to the H-repeat of Escherichia coli and to the trans-posases of a num ber of insertion sequence elements and has all the features of an inse rtion sequence element that has been designated VcIS1. Transposon inse rtion mutants defective in O139 O-antigen (and capsule) biosynthesis m ap to the same fragment as VcIS1. Preliminary sequence data of complem enting clones indicate that this DNA encodes a galactosyl-transferase and other enzymes for the utilization of galactose in polysaccharide b iosynthesis. We propose a mechanism by which both the Ogawa serotype o f O1 strains and the O139 serotype strains may have evolved.