Mutational analysis of the Shigella flexneri O-antigen polymerase Wzy: identification of Wzz-dependent Wzy mutants.

The O-antigen (Oag) component of lipopolysaccharide (LPS) is a major virulence determinant of Shigella flexneri and is synthesized by the O-antigen polymerase, WzySf. Oag chain length is regulated by chromosomally encoded WzzSf and pHS-2 plasmid-encoded WzzpHS2. To identify functionally important amino acid residues in WzySf, random mutagenesis was performed on the wzySf gene in a pWaldo-TEV-GFP plasmid, followed by screening with colicin E2. Analysis of the LPS conferred by mutated WzySf proteins in the wzySf-deficient (Δwzy) strain identified 4 different mutant classes, with mutations found in periplasmic loop 1 (PL1), PL2, PL3, and PL6, transmembrane region 2 (TM2), TM4, TM5, TM7, TM8, and TM9, and cytoplasmic loop 1 (CL1) and CL5. The association of WzySf and WzzSf was investigated by transforming these mutated wzySf plasmids into a wzySf- and wzzSf-deficient (Δwzy Δwzz) strain. Comparison of the LPS profiles in the Δwzy and Δwzy Δwzz backgrounds identified WzySf mutants whose polymerization activities were WzzSf dependent. Colicin E2 and bacteriophage Sf6c sensitivities were consistent with the LPS profiles. Analysis of the expression levels of the WzySf-GFP mutants in the Δwzy and Δwzy Δwzz backgrounds identified a role for WzzSf in WzySf stability. Hence, in addition to its role in regulating Oag modal chain length, WzzSf also affects WzySf activity and stability.