REGULATION OF NITROGEN FIXATION GENE EXPRESSION IN RHIZOBIA: AN OVERVIEW

Although nitrogen fixation is widely spread among prokaryotes, symbiotic nitrogen fixation by rhizobia is of special interest. First because of its major contribution to the nitrogen cycle and, second, as it takes place in a plant compartment, the nodule. There is thus a great deal of interest in understanding how the plant partner influences the nitrogen fixing activity of its microsymbiont and, conversely, how the microsymbiont adapts to the nodule environment and responds to its changing metabolic needs. This review will summarize our present knowledge of the genetic control underlying symbiotic nitrogen fixation in diverse Rhizobium genera. Special emphasis will be given to regulators. The role of oxygen as a key signal molecule in nitrogen fixation will be summarized and the likelihood of other signals operating on symbiotic nitrogen fixation will be discussed.