REGULATION OF THE EXPRESSION OF THE NOD BOX-INDEPENDENT NODULATION GENE, NOLX, IN SINORHIZOBIUM-FREDII, A NITROGEN-FIXING SYMBIONT OF LEGUME PLANTS

nolX, a nod box-independent nodulation gene from the nitrogen-fixing s ymbiont, Sinorhizobium fredii, regulates cultivar-specific nodulation of soybean. Expression of this gene is triggered by isoflavonoid cues from host roots but is not mediated by a typical cis-acting nod box pr omoter. We have found that two isoflavonoids, daidzein and coumestrol, induce expression of a nolX-lacZ gene fusion at pH values between 5.5 and 8.5, with an optimum at 6.5. Expression of nolX is highly depende nt on nutritional status but insensitive to salinity. Methionine and c asamino acids, amendments that enhance expression of hrpF, a homologou s gene from the plant pathogen Xanthomonas campestris biovar vesicator ia, have no and strongly negative effects, respectively, on expression of nolX. Growth of S. fredii on rhamnose, galactose, xylose, or myo;i nositol as carbon source greatly elevates inducibility in comparison t o a standard yeast extract-mannitol medium. None of the tested variabl es, however, released nolX from its dependence on isoflavonoids as ind ucers.