Ammonium sensing in nitrogen fixing bacteria: Functions of theglnB andglnD gene products

Abstract

A plentiful supply of fixed nitrogen as ammonium (or other compounds such as nitrate or amino acids) inhibits nitrogen fixation in free-living bacteria by preventing nitrogenase synthesis and/or activity. Ammonium and nitrate have variable effects on the ability of Rhizobiaceae (Rhizobium, Bradyrhizobium and Azorhizobium) species to nodulate legume hosts and on nitrogen fixation capacity in bacteroid cells contained in nodules or in plant-free bacterial cultures. In addition to effects on nitrogen fixation, excess ammonium can inhibit activity or expression of other pathways for utilization of nitrogenous compounds such as nitrate (through nitrate and nitrite reductase), or glutamine synthetase (GS) for assimilation of ammonium. This paper describes the roles of two key genes glnB and glnD, whose gene products sense levels of fixed nitrogen and initiate a cascade of reactions in response to nitrogen status. While work on Escherichia coli and other enteric bacteria provides the model system, glnB and, to a lesser extent, glnD have been studied in several nitrogen fixing bacteria. Such reports will be reviewed here. Recent results on the identity and function of the glnB and glnD gene products in Azotobacter vinelandii (a free-living soil diazotroph) and in Rhizobium leguminosarum biovar viciae, hereinafter designated R.l. viciae will be presented. New data suggests that Azotobacter vinelandii probably contains a glnB-like gene and this organism may have two glnD-like genes (one of which was recently identified and named nfrX). In addition, evidence for uridylylation of the glnB gene product (the PII protein) of R. l. viciae in response to fixed nitrogen deficiency is presented. Also, a glnB mutant of R. l. viciae has been isolated; its characteristics with respect to expression of nitrogen regulated genes is described.