The nodS Gene of Rhizobium tropici Strain CIAT899 Is Necessary for Nodulation on Phaseolus vulgaris and on Leucaena leucocephala. F. Waelkens. F. A. Janssens Laboratory of Genetics, Willem de Croylaan 42, B-3001 Heverlee, Belgium. T. Voets(1), K. Vlassak(2), J. Vanderleyden(1), and P. van Rhijn(1). (1) F. A. Janssens Laboratory of Genetics, Willem de Croylaan 42, B-3001 Heverlee, Belgium; (2) CNPAB/EMBRAPA, Km 47, Itaguai 23851, Rio de Janeiro, Brazil. MPMI 8:147-154. Accepted 19 October 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of The source. The American Phytopathological Society, 1995.

Rhizobium tropici strain CIAT899 induces nitrogen-fixing nodules on the roots of a wide range of tropical legumes, including Phaseolus vulgaris and Leucaena leucocephala. Previously, a DNA region of the CIAT899 pSym plasmid containing the common nodulation genes nodABC and one of the nodD alleles was characterized (P. van Rhijn, B. Feys, C. Verreth, and J. Vanderleyden, J. Bacteriol. 175: 438-447, 1993). As reported here, the region immediately downstream of nodC contains the nodSU genes. The nu-cleotide sequence of these genes is presented. CIAT899 nodS and nodU mutants were constructed. The nodS mutant was completely deficient in nodulation on the host plants P. vulgaris and L. leucocephala. The nodU mutation caused a decrease in nodulation on Leucaena but resorted no effect on Phaseolus. Introduction of the CIAT899 nodABCSU region in R. etli CE-3, a strain that only nodulates P. vulgaris, caused an extension of the host range of strain CE-3 to L. leucocephala.