ANALYSIS OF THE EXPRESSION FROM RHIZOBIUM-MELILOTI FIX-PROMOTERS IN OTHER RHIZOBIUM BACKGROUNDS

Using translational fusions to lacZ, we have measured expression from the promoters of rhizobium meliloti regulatory genes, nifA and fixK, a nd structural genes, nifH and fixA, in other fast-growing rhizobia who se nitrogen fixation regulation is less known. Neither nifA nor fixK p romoters were activated under both free-living microaerobic and symbio tic conditions, except in R. tropici, where clear symbiotic activation of either nifA or fixK expression could be observed. Both nifH and fi xA promoters showed strong heterologous activation during symbiosis an d weak activation under free-living nitrogen starvation conditions. On ly when the nifH promoter was in R. tropici and R. leguminosarum bv. p haseoli, was clear induction observed in the microaerobic free-living state. Deletion analysis of these promoters suggested that a NifA bind ing site (UAS) was needed for full heterologous activation of nifHp, e ither in microaerobiosis or symbiosis. In contrast, the UAS region see med to be unnecessary for fixA activation. However, a region containin g 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 sigma(54) holoenzyme recognition sequence; this may be indicative of the existence of non-specific activation by NtrC-like proteins. Ou r results suggest possible common and different features in the contro l mechanisms of the nitrogen fixation gene expression among Rhizobium species.