Analysis of the Rhizobium meliloti genes exoU, exoV, exoW, exoT, and exoI involved in exopolysaccharide biosynthesis and nodule invasion: exoU and exoW probably encode glucosyltransferases

Abstract

Sequence analysis of a 5.780-kb DNA fragment originating from megaplasmid 2 of Rhizobium meliloti 2011 involved in biosynthesis of exopolysaccharide I (EPS I) and invasion of alfalfa nodules revealed the presence of five exo genes designated exoU, exoV, exoW, exoT, and exoI. ExoT resembled transmembrane proteins, whereas ExoI displayed a characteristic signal peptide. Sequence comparisons with several polysaccharide-polymerizing enzymes of both prokaryotic and eukaryotic origin indicated that exoW and exoU encode glucosyltransferases. Moreover, ExoV displayed weak homologies to the ExoO, ExoA, ExoL, and ExoM proteins of R. meliloti, which are also discussed as glucosyltransferases. Using exo-lacZ transcription fusions in connection with plasmid integration mutagenesis, promoters were identified in front of exoI, exoT, exoW, exoV, and exoU.R. meliloti 2011 strains with mutations in exoT, exoW, exoV, and exoU produced no detectable EPS I and were unable to infect alfalfa nodules, whereas exoI mutants synthesized a reduced amount of EPS I and did infect alfalfa nodules.