Regulation of Nitrogen Fixation by the NIFL and NIFA Proteins from Azotobacter Vinelandii

Abstract

The extreme oxygen sensitivity of the nitrogenase component proteins and the high energetic requirements for nitrogen fixation impose physiological constraints on diazotrophy necessitating stringent regulation of nif gene transcription in response to fixed nitrogen and environmental oxygen (Hill, 1992). In most diazotrophs expression of genes required for the synthesis of molybdenum nitrogenase is positively controlled by the transcriptional activator NIFA, an enhancer binding protein which catalyses the formation of open promoter complexes by the holoenzyme form of RNA polymerase containing the alternative sigma factor σN (EσN) (Morett and Segovia 1993). Binding sites for NIFA, termed Upstream Activator Sequences (UAS) are usually located more than 100 base pairs upstream of the transcription start site and the activator appears to make contact with downstream bound EσNvia the formation of a DNA loop (Buck et al 1987). The formation of such loops is facilitated by the binding of an auxilliary protein, Integration Host Factor (IHF) which may stabilise these structures by correctly orientating the activator and polymerase binding sites and offsetting the bending energy required for DNA loop formation (Hoover et al 1990). Productive interactions between NIFA and EσN promote the transition from the closed promoter complex to the open promoter complex. An unusual feature of this reaction with respect to prokaryotic transcription systems is that it requires the hydrolysis of a nucleoside triphosphate, principally ATP or GTP (Popham et al 1989; Berger et al 1994; Austin et al 1994) (Fig 1)