Using translational fusions to lacZ, we have measured expression from the promoters of regulatory genes, and , and structural genes, and , in other fast-growing rhizobia whose nitrogen fixation regulation is less known. Neither nor promoters were activated under both free-living microaerobic and symbiotic conditions, except in , where clear symbiotic activation of either or expression could be observed. Both and promoters showed strong heterologous activation during symbiosis and weak activation under free-living nitrogen starvation conditions. Only when the promoter was in and bv. phaseoli, was clear induction observed in the microaerobic free-living state. Deletion analysis of these promoters suggested that a NifA binding site (UAS) was needed for full heterologous activation of , either in microaerobiosis or symbiosis. In contrast, the UAS region seemed to be unnecessary for fixA activation. However, a region containing a potential integration host factor (IHF) binding site was observed to be needed for complete heterologous symbiotic induction from fixAp. The moderate induction observed in nitrogen-free medium only required the holoenzyme recognition sequence; this may be indicative of the existence of non-specific activation by NtrC-like proteins. Our results suggest possible common and different features in the control mechanisms of the nitrogen fixation gene expression among Rhizobium species.


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