Regulation of nif Gene Expression in Free Living Diazotrophs: Recent Advances

Abstract

The major advances in our knowledge of nif gene regulation in recent years have been driven by the need to understand and characterise the components required for the activation of nif gene transcription and to elucidate the signal transduction pathways which transmit information concerning the oxygen/redox, carbon/energy and nitrogen status to the transcriptional apparatus. The genetic arrangement of regulatory genes is often different amongst diverse diazotrophs and regulatory circuits are somewhat varied. In addition, the modular domain structure of the regulatory proteins allows a certain level of fine tuning to adapt regulatory communication to the physiological requirements of the host organism. Nevetherless, it is clear that a common set of regulatory genes are utilized. The NIFA protein is a ubiquitous transcriptional activator in nitrogen-fixing proteobacteria and the nitrogen regulatory genes ntrC, ntrB, glnB and glnD apparently play a pervasive role. Each regulatory circuit ultimately leads to either regulation of NIFA expression or modulation of its activity in response to environmental or intracellular signals. We will only consider here the signals and components which regulate the activity of NIFA and the consequences for gene regulation.