Identification of an O -Acyltransferase Gene ( oacB ) That Mediates 3- and 4-O-Acetylation of Rhamnose III in Shigella flexneri O Antigens

O antigen (O polysaccharide) is an important and highly variable cell component present on the surface of cells which defines the serospecificity of Gram-negative bacteria. Most O antigens of Shigella flexneri, a cause of shigellosis, share a backbone composed of →2)-α-l-Rhap(III)-(1→2)-α-l-Rhap(II)-(1→3)-α-l-Rhap(I)-(1→3)-β-d-GlcpNAc-(1→ repeats, which can be modified by adding various substituents, giving rise to 19 serotypes. The known modifications include glucosylation on various sugar residues, O-acetylation on Rha(I), and phosphorylation with phosphoethanolamine on Rha(II) or/and Rha(III). Recently, two new O-antigen modifications, namely, O-acetylation at position 3 or 4 of Rha(III) and position 6 of GlcNAc, have been identified in several S. flexneri serotypes. In this work, the genetic basis for the 3/4-O-acetylation on Rha(III) was elucidated. Bioinformatic analysis of the genome of S. flexneri serotype 2a strain Sf301, which carries 3/4-O-acetylation on Rha(III), revealed an O-acyltransferase gene designated oacB. Genetic studies combined with O-antigen structure analysis demonstrated that this gene is responsible for the 3/4-O-acetylation in serotypes 1a, 1b, 2a, 5a, and Y but not serotype 6, which has a different O-antigen backbone structure. The oacB gene is carried by a transposon-like structure located in the proA-adrA region on the chromosome, which represents a novel mechanism of mobilization of O-antigen modification factors in S. flexneri. These findings enhance our knowledge of S. flexneri O-antigen modifications and shed light on the origin of new O-antigen variants.