IDENTIFICATION AND CHARACTERIZATION OF A RHIZOBIUM-LEGUMINOSARUM BV PHASEOLI GENE THAT IS IMPORTANT FOR NODULATION COMPETITIVENESS AND SHOWS STRUCTURAL HOMOLOGY TO A RHIZOBIUM-FREDII HOST-INDUCIBLE GENE

DNA sequence analysis of a 1.4-kb SalI-HindIII segment located approxi mately 2 kb upstream of the Rhizobium leguminosarum bv. phaseoli syrM gene revealed the presence of an open reading frame (ORF3) encoding a putative 295-amino acid polypeptide with a molecular mass of 33,401 Da . ORF3 is homologous to a R. fredii host-inducible gene. The proteins encoded by R. l. bv. phaseoli ORF3 and by the R. fredii host-inducible gene share 37% sequence identity. In contrast to the R. fredii host-i nducible gene, expression of ORF3 is not induced in the presence of Ph aseolus vulgaris root exudates or by specific flavonoids, able to indu ce nodulation genes in R. l. bv. phaseoli. A R. l. bv. phaseoli ORF3 m utant was constructed by site-directed deletion/replacement mutagenesi s. This mutant strain is not affected in symbiotic nitrogen fixation b ut exhibits a delay in nodulation on Phaseolus vulgaris. Moreover, thi s mutant was shown to be defective in competition for nodulation.