Characterization of Two Plasmid-borne lpsβ Loci of Rhizobium etli Required for Lipopolysaccharide Synthesis and for Optimal Interaction with Plants

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Abstract
In Rhizobium etli CFN42, both the symbiotic plasmid (pd) and plasmid b (pb) are required for effective bean nodulation. This is due to the presence on pb of a region (lpsβ) involved in lipopolysaccharide (LPS) biosynthesis. We report here the genetic array and functional features of this plasmid-borne region. The sequence analysis of a 3,595-bp fragment revealed the presence of a transcriptional unit integrated by two open reading frames (lpsβ1 and lpsβ2) essential for LPS biosynthesis and symbiosis. The lpsβ1 encodes a putative 193 amino acid polypeptide that shows strong homology with glucosyl-1P and galactosyl-1P transferases. The deduced amino acid sequence of the protein encoded by lpsβ2 was very similar to that of proteins involved in surface polysaccharide biosynthesis, such as Pseudomonas aeruginosa WpbM, Bordetella pertussis BplL, and Yersinia enterocolitica TrsG. DNA sequences homologous to lpsβ1 and lpsβ2 of R. etli CFN42 were consistently found in functionally equivalent plasmids of R. etli, R. leguminosarum bv. viciae, and R. leguminosarum bv. trifolii strains, but not in R. meliloti, R. loti, R. tropici, R. fredii, Bradyrhizobium, Azorhizobium, and Agrobacterium tumefaciens. Even though Rhizobium and Agrobacterium do not share lpsβ sequences, their presence is required for crown-gall tumor induction by R. etli transconjugants carrying the Ti plasmid.