Regulation of Biological Nitrogen Fixation

Abstract

Biological nitrogen fixation, a process found only in some prokaryotes, is catalyzed by the nitrogenase enzyme complex. Bacteria containing nitrogenase occupy an indispensable ecological niche, supplying fixed nitrogen to the global nitrogen cycle. Due to this inceptive role in the nitrogen cycle, diazotrophs are present in virtually all ecosystems, with representatives in environments as varied as aerobic soils (e.g., Azotobacter species), the ocean surface layer (Trichodesmium) and specialized nodules in legume roots (Rhizobium). In any ecosystem, diazotrophs must respond to varied environmental conditions to regulate the tremendously taxing nitrogen fixation process. All characterized diazotrophs regulate nitrogenase at the transcriptional level. A smaller set also possesses a fast-acting post-translational regulation system. Although there is little apparent variation in the sequences and structures of nitrogenases, there appear to be almost as many nitrogenase-regulating schemes as there are nitrogen-fixing species. Herein are described the paradigms of nitrogenase function, transcriptional control and post-translational regulation, as well as the variations on these schemes, described in various nitrogen-fixing bacteria. Regulation is described on a molecular basis, focusing on the functional and structural characteristics of the proteins responsible for control of nitrogen fixation.

Regulation of nitrogenase by 2-oxoglutarate-reversible, direct binding of a PII-like nitrogen sensor protein to dinitrogenase

“…By comparing the imidazole elutions, lack of ATP did not significantly affect the amount of dinitrogenase that copurified with the His-tagged NifI 2 , suggesting that the complex is stable in the absence of ATP. Elution with 10 mM 2OG instead of imidazole in the presence of ATP resulted in the elution of dinitrogenase because of its separation from NifI 1,2 , showing that the NifI 1,2 -NifDK complex was disrupted by 2OG. In the absence of ATP, however, much less dinitrogenase was eluted with 2OG, suggesting that ATP is required for 2OG to efficiently disrupt the NifI 1,2 -NifDK complex.…”

“…B iological nitrogen fixation is a key step in the global nitrogen cycle and is carried out by a variety of Bacteria and methanogenic Archaea (1). This process is catalyzed by the nitrogenase complex, which consists of dinitrogenase (an ␣ 2 ␤ 2 heterotetramer of the NifD and NifK proteins), the site of substrate reduction, and its specific electron-donor dinitrogenase reductase (a homodimer of the NifH protein) (2).…”

“…Nitrogen fixation is energetically demanding and highly regulated, often at multiple levels. Posttranslational regulation of nitrogenase activity in response to conditions such as addition of ammonium is termed switch-off and is present in some free-living diazotrophs (1). The best characterized mechanism of switch-off involves the reversible ADP-ribosylation of an arginine residue in NifH by the dinitrogenase reductase ADP ribosyltransferase͞dinitrogenase reductase-activating glycohydrolase (DraT͞DraG) system (3).…”

“…NifI 1 and NifI 2 form two distinct subfamilies of PII proteins, whereas all other known PII proteins, including GlnB and GlnK, form a third subfamily (26). The genes encoding these proteins are present in the nif operons of all nitrogen-fixing Archaea as well as some anaerobic nitrogen-fixing Bacteria, including Chlorobium tepidum, Dehalococcoides ethanogenes (27), Heliobacterium chlorum, and some members of the Clostridia and ␦-Proteobacteria (26).…”

Interactions between CCM and N2 fixation in Trichodesmium

“…On the post-translational level, part of the iron protein pool is modified to an inactive form in the afternoon, which can persist throughout the night, being activated again in the morning (Zehr et al 1993). The switch between active and inactive state of the iron protein is regulated by ADP-ribosylation via the activating enzyme dinitrogenase reductase-activating glycohydrolase (DRAG; Halbleib and Ludden 2000), which requires ATP and MnCl 2 in a specific ratio (Saari et al 1986). Both, the transcription of nitrogenase and the post-translational modification of its iron protein were shown to be controlled by an endogenous rhythm (Chen et al 1998).…”