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.