Shigella flexneri O-antigen is an important and highly variable cell component presented on the outer leaflet of the outer membrane. Most Shigella flexneri bacteria share an O-antigen 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 chemical groups to different sugars, giving rise to diverse O-antigen structures and, correspondingly, to various serotypes. The known modifications include glucosylation on various sugar residues, O-acetylation on Rha(I) or/and Rha(III), and phosphorylation with phosphoethanolamine on Rha(II) or/and Rha(III). Recently, a new O-antigen modification, namely, O-acetylation at position 6 of N-acetylglucosamine (GlcNAc), has been identified in S. flexneri serotypes 2a, 3a, Y, and Yv. In this study, the genetic basis of the 6-O-acetylation of GlcNAc in S. flexneri was elucidated. An O-acyltransferase gene designated oacD was found to be responsible for this modification. The oacD gene is carried on serotype-converting bacteriophage SfII, which is integrated into the host chromosome by lysogeny to form a prophage responsible for the evolvement of serotype 2 of S. flexneri. The OacD-mediated 6-O-acetylation also occurs in some other S. flexneri serotypes that carry a cryptic SfII prophage with a dysfunctional gtr locus for type II glucosylation. The 6-O-acetylation on GlcNAc confers to the host a novel O-antigen epitope, provisionally named O-factor 10. These findings enhance our understanding of the mechanisms of the O-antigen variation and enable further studies to understand the contribution of the O-acetylation to the antigenicity and pathogenicity of S. flexneri.

structure, O-antigen, Shigella flexneri, epitope, Serotypes, bacteriophage, acyltransferase

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
3,3,2	aLRhap	?	?	71.2	73.5	?	?
3,3	aLRhap
3,2	Ac
3	aLRhap
2	Ac
6	aDGlcp
	bDGlcpN	?	?	?	?	?	67.0


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
3,3,2	aLRhap	?	?	3.86	3.30	?	?
3,3	aLRhap
3,2	Ac
3	aLRhap
2	Ac
6	aDGlcp
	bDGlcpN	?	?	?	?	?	3.78-3.98


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
3,3,2	aLRhap	?/?	?/?	71.2/3.86	73.5/3.30	?/?	?/?
3,3	aLRhap
3,2	Ac
3	aLRhap
2	Ac
6	aDGlcp
	bDGlcpN	?/?	?/?	?/?	?/?	?/?	67.0/3.78-3.98

The spectrum also has 9 signals at unknown positions (not plotted).

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*O[C@H]1[C@H](O[C@H]2[C@H](O[C@@H]3[C@@H](O)[C@H](C)O[C@@H](O[C@H]4[C@H](O)[C@@H](CO[C@H]5O[C@H](CO)[C@@H](O)[C@H](O)[C@H]5O)O[C@@H](*)[C@@H]4NC(C)=O)[C@@H]3OC(C)=O)O[C@@H](C)[C@H](O)[C@H]2O)O[C@@H](C)[C@H](O)[C@H]1O

845.798 g/mol (C₃₄H₅₅R₂NO₂₃, R = next and previous repeats, not counted in mol weight)