IDENTIFICATION AND MOLECULAR ANALYSIS OF GLGS, A NOVEL GROWTH-PHASE-REGULATED AND RPOS-DEPENDENT GENE INVOLVED IN GLYCOGEN-SYNTHESIS IN ESCHERICHIA-COLI

The putative stationary-phase sigma factor (sigma(s)) encoded by rpoS is essential for glycogen synthesis, but is not required for the transcription of gIgC and gIgA, which encode ADP-glucose-pyrophosphorylase and glycogen synthase, respectively. Using a mini-Mu random chromosomal library and a screen for glycogen overproduction, we identified a novel gene (gIgS) involved in glycogen synthesis. gIgS maps at 66.6 min (3247 kb) on the chromosome and constitutes a monocistronic operon. It encodes a hydrophilic and highly charged small protein, with a molecular weight of 7886, which is strongly expressed in minicells. Experiments with single-copy chromosomal gIgS::IacZ gene fusions indicated that glgS expression is controlled by sigma(s) as well as by cAMP. Two transcriptional start sites were mapped in the upstream regulatory region of gIgS. The gIgSp1 transcript was absent in a cya mutant, whereas an rpoS mutant did not synthesize the gIgSp2 transcript. Although glycogen synthesis is strongly stimulated by overproduction of GlgS and is inhibited by a gIgS null mutation, gIgS does not affect the expression of the gIgCAP operon. Its potential role in the metabolic control of glycogen synthesis is discussed. Also, evidence is presented to show that the amount of glycogen accumulated in vivo in early stationary-phase cells is mainly determined by sigma(s)-controlled gene expression and allosteric activation of GlgC, whereas the absolute levels of expression of gIgCAP as well as the intracellular concentration of cAMP are of minor importance.