Symbiotic properties of rhizobia containing a flavonoid-independent hybrid nodD product.

A hybrid nodD gene consisting of 75% of the nodD1 gene of Rhizobium meliloti at the 5' end and 27% of the nodD gene of Rhizobium trifolii at the 3' end activates the six tested inducible nod promoters of Rhizobium leguminosarum, R. trifolii, or R. meliloti to maximal levels, even in the absence of flavonoids. In strains containing such a constitutive activating nodD gene, transcription of nod genes started at the same site as in flavonoid-induced strains containing a wild-type nodD gene. In contrast to heterologous wild-type nodD products, the constitutive activating nodD gene does not cause a limitation of the host range. Furthermore, R. leguminosarum, R. trifolii, and R. meliloti strains containing the constitutive activating nodD gene induce (pseudo) nodules on tropical leguminous plants. Comparison of the symbiotic properties of rhizobia containing the constitutive nodD hybrid gene with those of rhizobia containing various wild-type nodD genes indicates that the activation of the nodD product by flavonoids is of crucial importance during the process of infection thread formation and, surprisingly, also during nitrogen fixation.