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. Expression of the fructose operon (fruBKA) encoding FruB (the diphosphoryl transfer protein), FruK (fructose 1-phosphate kinase) and FruA (the fructose permease) is negatively controlled by a transcription factor originally called the fructose repressor or FruR.
It was proposed by M. Saier, Jr. and coworkers 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 [JB].
However, the negative transcriptional regulation exerted by FruR on the glucose PTS genes [JBC] does not seem to be physiologically relevant, as indicated by measurements of glucose uptake rates in glucose-grown fruR mutant strains [JB]. Also, the repressive effect of FruR on transcription of the pykF gene encoding pyruvate kinase [JB] and pfkA encoding phosphofructokinase [JB], and subsequent effects on the Embden-Meyerhof-Parnas (EMB) pathway were not confirmed [JB].
As regards the control of FruR by different catabolites, data indicate that the transcriptional regulation by FruR is mainly dependent on the presence of intracellular fructose 1-phosphate. Indeed it has not been established that catabolites other than fructose 1-phosphate can act on FruR in vivo. In vitro, transcriptional regulation by FruR was not affected by catabolites other than fructose-1-phosphate and much less effectively fructose-1, 6-diphosphate [Journal of Molecular Biology]. Micromolar concentrations of fructose-1-phosphate as opposed to millimolar concentrations of fructose-1, 6-diphosphate displaced FruR from operator sites of FruR-dependent genes [Research in Microbiology].
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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