Regulation of nitrogen fixation in diazotrophs: the regulatory nifA gene and its characteristics

Biological nitrogen fixation is an important link in the nitrogen cycle. The ability to convert atmospheric nitrogen to metabolically usable compounds is confined to a few classes of bacteria and blue-green algae. Nitrogen-fixing organisms can be divided into two groups, (i) The free-living species that fix nitrogen for their own purpose, e.g. Klebsiella pneumoniae and Azotobacter vinelandii. (ii) The species that depend on an intimate association with plants, e.g. the cyanobacterium Anabaena azollae with the waterfern Azolla, or on intracellular symbiosis, e.g. the bacterial genera Rhizobium, Bradyrhizobium and Azorhizobium with various, specific leguminous host plants, and the actinomycete Frankia with alders. Nitrogen fixation by Rhizobium species in symbiosis with members of the plant family Leguminosae [amongst others: pea, ( Pisum sativum L.), soybean ( Glycine max L.) and alfalfa ( Medicago sativa L.)] is of great importance to agriculture. The interaction of bacteria and plants is a very complicated multistep process that involves gene products from both symbiotic partners. Rhizobium bacteria interact with their specific host plants, which results in the formation of newly developed anatomical structures on the main and lateral roots of the plants, the root nodules. In some cases, e.g. Azorhizobium caulinodans, with its host plant Sesbania rostrata, nodules are also formed on the stem. In the nodules some cells contain bacteroids, specialized forms of the bacterium that are capable of fixing nitrogen by means of the oxygen-sensitive enzyme, nitrogenase. This enzyme complex, which consists of three different polypeptides, fixes atmospheric nitrogen into ammonia. The symbiotic bacteria-plant interaction is marked by the symbiosis-specific expression of both bacterial and plant genes. In this review we will summarize and discuss data specifically concerning the regulation of the genes that are expressed in nitrogen-fixing bacteria. We will focus upon two bacterial species that are exemplary: the facultative anearobic K. pneumoniae and the obligate aerobe Rhizobium meliloti, the symbiont of alfalfa ( M. sativa).