The fructose repressor of Escherichia coli

In Escherichia coli, fructose is transported and phosphorylated to fructose-1-phosphate in a reaction involving the phosphotransferase system (PTS).  Expression of the fructose operon (fruBKA) encoding FruB (the diphosphoryl transfer protein), FruK (fructose-1-phosphate kinase) and FruA (the fructose permease renamed Enzyme IIB'BCFru) is negatively controlled by a transcription factor originally called the fructose repressor or FruR [FruR at UniProtKB].

It was proposed by Saier and Ramseier that FruR should be renamed Cra for Catabolite repressor/activator protein.  This proposal was based upon the discovery that (i) FruR is a pleiotropic regulator controlling synthesis of enzymes involved in carbon metabolism and (ii) several catabolites are controlling the transcriptional regulation by FruR [J Bacteriol].

However, the negative transcriptional regulation exerted by FruR on the glucose PTS genes [J Biol Chem] does not seem to be physiologically relevant, as indicated by measurements of glucose uptake rates in glucose-grown fruR mutant strains [J Bacteriol].  Also, the repressive effect of FruR on transcription of the pykF gene encoding pyruvate kinase [J Bacteriol] and pfkA encoding phosphofructokinase [J Bacteriol], and subsequent effects on the Embden-Meyerhof-Parnas (EMB) pathway were not confirmed [J Bacteriol].

As regards the control of FruR by different catabolites, data indicated that the transcriptional regulation by FruR is mainly dependent on the presence of intracellular fructose-1-phosphate.  Indeed it was not been established that catabolites other than fructose-1-phosphate can act on FruR in vivoIn vitro, transcriptional regulation by FruR was not affected by catabolites other than fructose-1-phosphate and much less effectively fructose-1, 6-bisphosphate [J Mol Biol].  Micromolar concentrations of fructose-1-phosphate as opposed to millimolar concentrations of fructose-1, 6-bisphosphate displaced FruR from operator sites of FruR-dependent genes [Res Microbiol].  It is to be noted that the concentration of fructose-1, 6 bisphosphate was reported to be in the micromolar range with a carbon source such as acetate.  In Pseudomonas putida, fructose-1-phosphate is the only physiological effector of FruR however the reported in vitro effect of fructose-1, 6-bisphosphate on E. coli FruR was not definitely ruled out [FEBS Open Bio].

In addition, the FruR-mediated regulation of adenylate cyclase depends on the rapid uptake of fructose and is observed only in fructose-grown cells [Microbiology].  Should Cra [cra] be renamed FruR [fruR]?

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