(now is a symbiotic nitrogen-fixing bacterium that can nodulate soybean in a cultivar-specific manner. This process is governed by a set of negatively acting nodulation genes termed These genes prevent strain USDA257 from infecting soybean cultivars such as McCall, but they do not block nodulation of cultivar Peking. strain USDA191 contains DNA sequences that hybridize to yet it forms normal nitrogen-fixing nodules on both McCall and Peking soybean. These sequences were isolated and their structure and function examined in comparison to of strain USDA257. Restriction maps of the two loci are identical, as is a 2.4 kb DNA sequence that corresponds to and its promoter region. Expression of by strain USDA191 is flavonoid-dependent in culture and readily detectable in nodules. The gene is not inducible in a mutant of strain USDA191 that lacks the regulatory gene, and its expression is greatly attenuated in a mutant. is also present and flavonoid-inducible in HH103, a second strain that nodulates McCall soybean normally. Inactivation of in strain HH103, USDA191 or USDA257 leads to retardation of initial nodulation rates on soybean cultivars such as Peking and to acquisition of the capacity to form nitrogen-fixing nodules on two species of is thus of symbiotic significance in all three strains, even though it regulates soybean cultivar specificity only in strain USDA257.


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