Regulation of ammonium assimilation in Haloferax mediterranei: interaction between glutamine synthetase and two GlnK proteins.

GlnK proteins belong to the PII superfamily of signal transduction proteins and are involved in the regulation of nitrogen metabolism. These proteins are normally encoded in an operon together with the structural gene for the ammonium transporter AmtB. Haloferax mediterranei possesses two genes encoding for GlnK, specifically, glnK(1) and glnK(2). The present study marks the first investigation of PII proteins in haloarchaea, and provides evidence for the direct interaction between glutamine synthetase and both GlnK(1) and GlnK(2). Complex formation between glutamine synthetase and the two GlnK proteins is demonstrated with pure recombinant protein samples using in vitro activity assays, gel filtration chromatography and western blotting. This protein-protein interaction increases glutamine synthetase activity in the presence of 2-oxoglutarate. Separate experiments that were carried out with GlnK(1) and GlnK(2) produced equivalent results.