Molecular characterization of the genes involved in O-antigen modification, attachment, integration and excision in Shigella flexneri bacteriophage SfV.

Bacteriophage SfV is a temperate phage of Shigella flexneri responsible for converting serotype Y (3,4) to serotype 5a (V; 3,4) through its glucosyl transferase gene. The glucosyl transferase (gtr) gene of SfV has been cloned and shown to partially convert S. flexneri serotype Y to serotype 5a. In this study, we found that the serotype-converting region of SfV was approximately 2.5 kb in length containing three continuous ORFs. The recombinant strain carrying the three complete ORFs expressed the type V and group antigen 3,4, both indistinguishable from that of S. flexneri 5a wild-type strain. The interruption of orf5 or orf6 gave partial conversion in the S. flexneri recombinant strain indicated by the incomplete replacement of group antigen 3,4. The region adjacent to the serotype-conversion genes was found to be identical to the attP-int-xis region of phage P22. Altogether, an approximately 2.2-kb sequence covering a portion of the serotype-conversion (approximately 500 nt)-attP-int-xis regions of SfV was remarkably similar to that of P22.