THE SOYBEAN CULTIVAR SPECIFICITY GENE NOLX IS PRESENT, EXPRESSED IN ANODD-DEPENDENT MANNER, AND OF SYMBIOTIC SIGNIFICANCE IN CULTIVAR-NONSPECIFIC STRAINS OF RHIZOBIUM (SINORHIZOBIUM) FREDII

Rhizobium (now Sinorhizobium) fredii is a symbiotic nitrogen-fixing ba cterium that can nodulate soybean in a cultivar-specific manner. This process is governed by a set of negatively acting nodulation genes ter med noIXWBTUV. These genes prevent R. fredii strain USDA257 from infec ting soybean cultivars such as McCall, but they do not block nodulatio n of cultivar Peking. R. fredii strain USDA191 contains DNA sequences that hybridize to noIXWBTUV, yet it forms normal nitrogen-fixing nodul es on both McCall and Peking soybean. These sequences were isolated an d their structure and function examined in comparison to noIXWBTUV of strain USDA257. Restriction maps of the two loci are identical, as is a 2.4 kb DNA sequence that corresponds to noIX and its promoter region . Expression of noIX by strain USDA191 is flavonoid-dependent in cultu re and readily detectable in nodules. The gene is not inducible in a m utant of strain USDA191 that lacks the regulatory nodD1 gene, and its expression is greatly attenuated in a nodD2 mutant. noIX is also prese nt and flavonoid-inducible in HH103, a second R. fredii strain that no dulates McCall soybean normally. Inactivation of noIX in strain HH103, USDA191 or USDA257 leads to retardation of initial nodulation rates o n soybean cultivars such as Peking and to acquisition of the capacity to form nitrogen-fixing nodules an two species of Erythrina. noIX is t hus of symbiotic significance in all three strains, even though it reg ulates soybean cultivar specificity only in strain USDA257.