Relaxed Sugar Donor Selectivity of a Sinorhizobium meliloti Ortholog of the Rhizobium leguminosarumMannosyl Transferase LpcC: ROLE OF THE LIPOPOLYSACCHARIDE CORE IN SYMBIOSIS OFRHIZOBIACEAE WITH PLANTS*
The lpcC gene of Rhizobium leguminosarum and the lpsB gene ofSinorhizobium meliloti encode protein orthologs that are 58% identical over their entire lengths of about 350 amino acid residues. LpcC and LpsB are required for symbiosis with pea andMedicago plants, respectively. S. meliloti lpsBcomplements a mutant of R. leguminosarum defective inlpcC, but the converse does not occur. LpcC encodes a highly selective mannosyl transferase that utilizes GDP-mannose to glycosylate the inner 3-deoxy-d-manno-octulosonic acid (Kdo) residue of the lipopolysaccharide precursor Kdo2-lipid IVA. We now demonstrate that LpsB can also efficiently mannosylate the same acceptor substrate as does LpcC. Unexpectedly, however, the sugar nucleotide selectivity of LpsB is greatly relaxed compared with that of LpcC. Membranes of the wild-type S. meliloti strain 2011 catalyze the glycosylation of Kdo2-[4′-32P]lipid IVA at comparable rates using a diverse set of sugar nucleotides, including GDP-mannose, ADP-mannose, UDP-glucose, and ADP-glucose. This complex pattern of glycosylation is due entirely to LpsB, since membranes of the S. meliloti lpsB mutant 6963 do not glycosylate Kdo2-[4′-32P]lipid IVA in the presence of any of these sugar nucleotides. Expression oflpsB in E. coli using a T7lacpromoter-driven construct results in the appearance of similar multiple glycosyl transferase activities seen in S. meliloti 2011 membranes. Constructs expressing lpcC display only mannosyl transferase activity. We conclude that LpsB, despite its high degree of similarity to LpcC, is a much more versatile glycosyltransferase, probably accounting for the inability of lpcC to complementS. meliloti lpsB mutants. Our findings have important implications for the regulation of core glycosylation in S. meliloti and other bacteria containing LpcC orthologs.
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Published, JBC Papers in Press, February 17, 2003, DOI 10.1074/jbc.M301256200
This work was supported by National Institutes of Health Grants R37-GM-51796 (to C. R. H. R.) and GM54882 (to R. J. C.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
