ISOLATION AND CHARACTERIZATION OF A GENE FOR NODULE DEVELOPMENT LINKED TO THE NDVA AND NDVB GENES IN RHIZOBIUM SP STRAIN TAL1145

Six overlapping cosmid clones containing genes for nodule development were isolated from Rhizobium sp. strain TAL1145 that nodulates both Ph aseolus bean and the tree legume leucaena. These clones complemented a Rhizobium etli mutant, MLC640, for defects in nodule development and motility. Two of these clones hybridized with chromosomal fragments co ntaining the ndvA gene from R. meliloti and the ndvB gene from R. fred ii, required for nodule development in alfalfa and soybean, respective ly. A Tn5-insertion mutant, SP571, was constructed from TAL1145 by sit e-directed mutagenesis using one of these clones. SP571 formed nitroge n-fixing nodules on leucaena but non-fixing nodules on beans. The Tn5 insertion in this mutant was located in a 2.1-kb EcoRI fragment, appro ximately 15 kb away from the fragment that hybridized with the ndvA an d ndvB genes. The 2.1-kb fragment, cloned in a wide-host-range vector, complemented the mutant SP571 for nitrogen fixation on beans.