Diversity of Nitrogenase Systems in Diazotrophs

Abstract

Nitrogenase is a metalloprotein complex that catalyses the reaction of biological nitrogen fixation. At least three genetically distinct nitrogenase systems have been confirmed in diazotrophs, namely Nif, Vnf, and Anf, in which the active-site central metals are Mo, V, and Fe, respectively. The present review summarizes progress on the genetic, structural, and functional investigations into the three nitrogenases and discusses the possibility of the existence of other novel nitrogenases. Key words: diazotroph; FeMo-cofactor and cofactors substituted for Mo; nitrogenase system; P-cluster. Zhao Y, Bian SM, Zhou HN, Huang JF (2006). Diversity of nitrogenase systems in diazotrophs. J Integrat Plant Biol 48(7), 745−755.www.blackwell-synergy.com; www.jipb.net . Review .Biological nitrogen fixation is an essential step in the global nitrogen cycle and this reaction is necessary to sustain life on earth (Seefeldt et al. 2004). Nitrogen fixation is mediated by the enzyme system nitrogenase, which, although found in a relatively few groups of prokaryotes, is responsible for the cycling of some 10 8 tons of N per year (Eady et al. 1996(Eady et al. , 2003. In general, the nitrogenase enzyme is composed of two oxygenlabile and separable metalloproteins, dinitrogenase (component I) and dinitrogenase reductase (component II). Component I contains the active site for N 2 reduction, with a molecular weight of approximately 240 kDa and is composed of two heterodimers. Component II is a 60-70 kDa homodimer coupling ATP hydrolysis to interprotein electron transfer (Zehr et al. 2003). At least three genetically distinct nitrogenase systems have been identified until now: Nif, Vnf, and Anf. Apart from the three O 2 -sensitive nitrogenases, a third alternative, but completely nif/ vnf/anf-independent and O 2 -insensitive nitrogenase system, has b e e n r e p o r t e d r e c e n t l y t o o c c u r i n S t r e p t o m y c e s thermoautotrophicus (Ribbe et al. 1997). Many studies have been conducted focusing on the structure and mechanism of nitrogenase systems. In recent years, considerable progress has been made towards establishing the mechanism of the action of nitrogenase complex, reflecting the contribution of a combination of structural, biochemical, spectroscopic, synthetic, and theoretical approaches to a challenging problem, with implications for a range of biochemical and chemical systems (Rees and Howard 2000).In the present review, we focus on the genetics, molecular biology and the structure and function of alternative (Moindependent) nitrogenases. The possibility of the existence of some novel nitrogenases is also discussed.

Microbial consortia of bacteria and fungi with focus on the lichen symbiosis

“…Nitrogenases are ubiquitous in Alphaproteobacteria, but nitrogen fixation could also be accomplished by fungal-associated bacteria of other groups, notably Betaproteobacteria (Leveau and Preston, 2008). Nitrogen-fixing systems are also known from other bacterial groups involved in symbiotic relations (Kneip et al, 2007), and can also include alternative nitrogenase systems (Zhao et al, 2006). Growth of isolated strains on N-free media (Cardinale et al, 2006;Grube et al, 2009) and the results from acetylene-reduction assays by Liba et al (2006) provided support for nitrogen fixation being undergone by lichen-associated bacteria.…”

The lime–silicate question

“…In some microorganisms, the element Mo is substituted by vanadium or they express a nitrogenase that contains neither of these elements (Zhao et al, 2006). Such organisms with these alternative nitrogenases could have a selective advantage in soils where Mo is limiting.…”

Nitrogen Fixation and Hydrogen Metabolism in Cyanobacteria

“…The discovery of the alternative nitrogenases without molybdenum in their prosthetic groups can be regarded as a milestone in nitrogenase research. Reviews on this subject are available (18,59,167,242). The aerobe Azotobacter vinelandii possesses gene sets for all three different types of nitrogenases (Fig.…”