Regulation of Expression of the gabDTP Operon and Utilization of GABA as a Sole Nitrogen Source in Escherichia coli

Abstract

<p>The <em>gabDTP</em> operon is regulated by a number of different signals and stress conditions including general stress via σ^s, carbon limitation, cell-to-cell signaling, alkaline pH and nitrogen limitation. The mechanism of regulation by many of these conditions is unclear. Operon <em>lacZ</em>-fusions to the genes of the <em>gabDTP</em> operon, isolated in a previous search for <em>rpoS</em>-dependent genes in <em>E. coli</em>, showed a 10-fold <em>rpoS</em>-dependence in rich LB media. Here we provide evidence, utilizing Northern analysis of wildtype <em>E. coli</em> strains without any insertions in the <em>gabDTP</em> operon, that expression of the <em>gabDTP</em> operon was highly RpoS- and stationary phase-dependent. The corresponding enzymes GabD (SSDH) and GabT CGSST) showed a 20 fold higher activity in stationary phase cultures of wildtype cells compared to isogenic<em> rpoS</em>-mutant strains. However, contrary to expected results, growth on GABA as a sole nitrogen source is better in <em>rpoS</em>^- mutants relative to wildtype strains. Similar growth patterns are observed when the same strains are grown on L-glutamate as the sole nitrogen source, suggesting that the limitation for growth with GABA as the nitrogen source may be due to an inability to utilize glutamate efficiently as a sole nitrogen source. These results confirm that the <em>gabDTP</em> operon is part of the RpoS regulon, and suggest the metabolism of GABA plays a role in stationary phase stress resistance in <em>E. coli</em>.</p>