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J. Biol. Chem., Vol. 278, Issue 18, 16365-16371, May 2, 2003

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Relaxed Sugar Donor Selectivity of a Sinorhizobium meliloti Ortholog of the Rhizobium leguminosarum Mannosyl Transferase LpcC
ROLE OF THE LIPOPOLYSACCHARIDE CORE IN SYMBIOSIS OF RHIZOBIACEAE WITH PLANTS^*

Margaret I. Kanipes, Suzanne R. Kalb^§, Robert J. Cotter^§, Daniela F. Hozbor^¶, Antonio Lagares^¶**, and Christian R. H. Raetz

From the  Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, the ^§ Department of Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, and ^¶ Instituto de Bioquímica y Biología Molecular, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, La Plata 1900, Argentina

The lpcC gene of Rhizobium leguminosarum and the lpsB gene of Sinorhizobium 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 and Medicago plants, respectively. S. meliloti lpsB complements a mutant of R. leguminosarum defective in lpcC, 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 IV_A. 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 IV_A 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 IV_A in the presence of any of these sugar nucleotides. Expression of lpsB in E. coli using a T7lac promoter-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 complement S. 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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