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-lnducible Gene. J. Michiels. F. A. Janssens Laboratory of Genetics, Catholic University of Leuven, Willem de Croylaan 42, B-3001 Hev- erlee, Belgium. H. Pelemans (1), K. Vlassak (1,2), Verreth (1), and J. Vanderleyden (1). (1)F.A. Janssens Laboratory of Genetics, Catholic University of Leuven, Willem de Croylaan 42, B-3001 Hev- erlee, Belgium; (2)CNPAB/EMBRAPA, Km 47, Itaguai 23851, Rio de Janeiro, Brazil. MPMI 8: 468-472. Accepted 24 January 1995. Copyright 1995 The American Phytopathological Society.

DNA sequence analysis of a 1.4-kb SalI-HindIII segment located approximately 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 polypeplide with a molecular mass of 33,401 Da. ORF3 is homologous to a R. fredii host-inducible gene. The proteins encoded by R. I. bv. phaseoli ORF3 and by the R. fredii host-inducible gene share 37% sequence identity. In contrast to the R. fredii host-inducible gene, expression of ORF3 is not induced in the presence of Phaseolus vulgaris root exudates or by specific flavonoids, able to induce nodulation genes in R. I. bv. phaseoli. A R. I. bv. phaseoli ORF3 mutant was constructed by site-directed deletion/replacement mutagene-sis. This mutant strain is not affected in symbiotic nitrogen fixation but exhibits a delay in nodulation on Phaseolus vulgaris. Moreover, this mutant was shown to be defective in competition for nodulation