Type IV O antigen modification genes in the genome of Shigella flexneri NCTC 8296

The genes encoding type IV O antigen glucosylation were characterized from both Escherichia coli and Shigella flexneri. The putative O antigen modification genes from E. coli, o120 o306 o443, were PCR-amplified and introduced into S. flexneri serotype Y strain SFL124. Immunogold labelling and phage sensitivity indicated the presence of both serotype Y and serotype 4a O antigens on the cell surface of the resulting recombinant SFL124 strains, suggesting that only partial serotype conversion was conferred by the E. coli genes. The type IV O antigen modification genes were then isolated and characterized from S. flexneri serotype 4a strain NCTC 8296. A 3.8 kb chromosomal fragment conferred complete conversion to serotype 4a when introduced into SFL124. Sequence analysis of the fragment revealed the presence of three genes, gtrA(IV) gtrB(IV) gtrIV(Sf). DNAs homologous to bacteriophage int and attP were located upstream of gtrA(IV), suggesting that this region of the NCTC 8296 genome may have originated from a bacteriophage; however, a serotype-converting phage could not be induced from this strain nor from other strains used in this study. Comparison of the GtrIV(Sf) and GtrIV(Ec) (o443) proteins revealed that they are 41% identical and 63% similar, which is the highest degree of similarity reported among the S. flexneri O antigen glucosyltransferases.