Small virulence plasmid of Shigella dysenteriae 1 strain W30864 encodes a 41,000-dalton protein involved in formation of specific lipopolysaccharide side chains of serotype 1 isolates.

A 6-megadalton plasmid, pHW400, of Shigella dysenteriae 1 strain W30864 was previously found to specify one or more functions for O-antigen production and bacterial virulence (H. Watanabe and K. N. Timmis, Infect. Immun. 43:391-396, 1984). The region of pHW401, a Tn801-tagged derivative of pHW400, responsible for O-antigen production has been localized by gene cloning and Tn5 transposon mutagenesis. Analysis of lipopolysaccharide isolated from S. dysenteriae 1 bacteria carrying mutant plasmids revealed that the determinant for O-antigen synthesis, designated rfp, codes for a function involved in the formation of the O-polysaccharide side chain structure of lipopolysaccharide. Analysis of radioactively labeled proteins synthesized in minicells of Escherichia coli carrying mutant plasmids identified the product of the rfp gene as a 41,000-dalton protein. Southern hybridization with a DNA fragment carrying the rfp gene demonstrated that this determinant is present on 6-megadalton plasmids in other isolates of S. dysenteriae 1 but is not present at all in a variety of other Shigella, E. coli, and Salmonella typhimurium strains that were tested.