Identification of nodX, a gene that allows Rhizobium leguminosarum biovar viciae strain TOM to nodulate Afghanistan peas

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Abstract
Gene(s) conferring the ability of Rhizobium leguminosarum biovar viciae strain TOM to nodulate primitive peas (cultivar Afghanistan) had been located in a 2.0 kb region of its sym plasmid, pRL5JI. In this DNA, a single open reading frame of 1101 bp, corresponding to a gene, nodX was found. nodX is downstream of nodJ which is present in strain TOM and also in the sym plasmid of a typical strain of this biovar. nodX specifies a hydrophobic protein (of Mr 41 036) with no clear similarity to other proteins in data bases. Mutations in nodX abolished nodulation of Afghanistan peas but not nodulation of commercial peas. nodX-lacZ fusions were used to show that transcription of nodX was activated by root exudates from both commercial and Afghanistan peas and by defined flavonoids. Exudate from Afghanistan peas activated nod genes of typical strains of R. leguminosarum biovar viciae which fail to nodulate these peas; thus, their failure to nodulate these primitive peas is not due to a lack of activation of their nod genes by exudate from Afghanistan peas. A homologue of nodX exists in R. leguminosarum biovar trifolii (which nodulates clover) but not in typical strains of R. leguminosarum biovar viciae.