The biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen (N2) to ammonium by nitrogenase enzyme and is an exclusive property of prokaryotes. Nitrogen fixation is a key step in the nitrogen cycle as is the most important way N2 from atmosphere enters biological systems. Nitrogen fixing organisms (diazotrophs) are found in Bacteria and in some groups of Archaea. All known diazotrophs contain at least one of the three related sub-types of nitrogenase: Nif, Vnf, and Anf. The best studied sub-type is the molybdenum-dependent nitrogenase - Nif. The organization and regulation of nif genes are relatively well characterized in Gram negative but not in Gram positive diazotrophs. Interestingly, the Gram-positive bacteria show contrasting properties in their motifs of nif gene promoters and arrangement compared to the Gram negative bacteria. There is no gene similar to the nifA for transcriptional regulation in Gram positive diazotrophs, but nitrogenase synthesis is also subject to the availability of ammonia in the environment. Although nifHDK sequences are present in those bacteria, the other genes grouped with nifHDK and the regulatory mechanisms for BNF expression is still not elucidated in Gram positive bacteria. This review will focus on BNF, nitrogen fixing genes and their distribution in Gram positive bacteria.
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