Analysis of the Rhizobium meliloti exoH/exoK/exoL fragment: ExoK shows homology to excreted endo-β-1,3-1,4-glucanases and ExoH resembles membrane proteins

Nucleotide sequencing of a 4.15 kb DNA fragment from megaplasmid 2 of Rhizobium meliloti 2011 revealed the location of the genes exoH, exoK and exoL. The putative proteins encoded by these genes have molecular weights of 41, 30, and 44 kDa, respectively. The hydrophobicity profile of the ExoH amino acid sequence resembles that of transmembrane proteins. The predicted exoL gene product does not contain hydrophobic regions, indicating a cytoplasmic localization. The exoK gene product is characterized by a putative signal peptide and exhibits significant homology to endo-β-1,3 1,4-glucanases of bacilli and Clostridium thermocellum. R. meliloti exoK mutants induced pink nodules and synthesized a reduced amount of exopolysaccharide (EPS). Colonies of this mutant showed a delay in the appearance of the Calcofluor white fluorescence. In addition, the formation of the characteristic halo was strongly delayed. R. meliloti exoL and exoH mutants induced pseudonodules. The exoH, but not the exoL mutant, synthesized an EPS that could be precipitated by cetyl pyridinium chloride (CPC) and also by ethanol. Plasmid integration mutagenesis revealed promoter regions preceding exoH, exoK and exoL.