Genetic analysis of the Rhizobium meliloti exoYFQ operon: ExoY is homologous to sugar transferases and ExoQ represents a transmembrane protein.

The nucleotide sequence of a 4.8-kb ClaI-EcoRI DNA fragment of megaplasmid 2 of Rhizobium meliloti Rm2011 involved in succinoglucan (EPS I) synthesis and nodule infection was determined. Four open reading frames (ORFs) were identified on this fragment. A mutational analysis revealed that these ORFs represent genes that were termed exoX, exoY, exoF, and exoQ. The locations of transposon insertions in these exo genes were determined at the nucleotide level. Plasmid integration mutagenesis revealed that the genes exoY, exoF, and exoQ are organized in an operon. The exoX gene running in opposite direction forms a monocistronic transcriptional unit. The exoX gene was shown to negatively influence the amount of EPS I synthesized. The exoY gene is coding for a membrane associated protein homologous to the C-terminal part of the Xanthomonas campestris glucosyltransferase GumD and the Salmonella typhimurium galactose transferase RfbP. ExoF, a probable periplasmatic protein, is nearly identical to the protein encoded by ORF1 of Rhizobium sp. strain NGR234. ExoQ is most probably a membrane associated protein as deduced by its hydrophobic structural features. All three genes of the exoYFQ operon were shown to be essential for succinoglucan synthesis and nodule infection.