Glycerol-3-phosphate-mediated repression of malT in Escherichia coli does not require metabolism, depends on enzyme IIAGlc and is mediated by cAMP levels

malT encodes the central activator of the maltose system in Escherichia coli, a gene that is typically under positive control of the cAMP/CAP catabolite repression system. When cells were grown in tryptone broth, the addition of glycerol reduced malT expression two- to threefold. Phosphorylation of glycerol to glycerol-3-phosphate (G3P) was necessary for this repression, but further metabolism to dihydroxyacetone phosphate was not. Mutants lacking adenylate cyclase and harbouring a crp* mutation (synthesizing a cAMP receptor protein that is independent of cAMP) no longer repressed a transcriptional malT-lacZ fusion but still repressed a translational malT-lacZ fusion. Similar results were obtained with a mutant lacking enzyme IIA(Glc). For the translational fusion (in a cya crp* genetic background) to be repressed by glycerol, a drop to pH 5 of the growth medium was necessary. Thus, while transcriptional repression by glycerol requires enzyme IIA(Glc), cAMP and CAP, pH-mediated translational repression is cAMP independent. Other sugars that are not transported by the phosphotransferase system, most notably D-xylose, showed the same effect as glycerol.