Studies in this program are focused in two areas: (a) regulation of expression of R. meliloti nitrogen fixation (nif) genes and (b) characterization of two symbiotically essential loci (ndv) and ndvB) required for nodule invasion in R. meliloti. Both are continuations of ongoing research and represent efforts to examine aspects of early and late stages of the developmental interaction between a Rhizobium species and its host legume. Work on nif regulation will deal almost exclusively with molecular controls exerted upon and by the nifA gene. As the central regulatory element governing nif transcription, nifA has recently been shown to be the point at which regulation differs in Rhizobium from that observed in the free living nitrogen fixing bacterium Klebsiella pneumoniae. Techniques have been developed that allow study nif regulation in R. meliloti in the absence of alfalfa and this capability has been used to identify a new symbiotically essential locus controlling nifA expression. This locus will be part of the present study. Structural and regulatory features of ndv genes will be investigated, as will pertinent aspects of the ndv mutant phenotype. The overall objective is to ascertain the biochemical nature of the defect in ndv mutants and thus the normal function of ndv gene products. Of particular importance will be studies related to the absence of a unique polysaccharide (B-l,2 glucan) in ndv mutants and how this could relate to the decreased ability of ndv mutants to attach to and invade alfalfa roots.
