Chapter IV: Regulation of adenylate cyclase by glucose 6-phosphate transport
Glucose 6-phosphate is transported into the cell via the Uhp system (Uptake hexose phosphate), which is inducible by extracellular glucose 6-phophate. The Uhp system consists of a permease UhpT (encoded by uhpT), whose transcription is regulated by the UhpABC system. UhpA and UhpB belong to the family of 'two-component regulatory systems' (Robinson VL, 2000). In the presence of external glucose 6-phosphate, membrane-bound UhpC interacts with membrane-bound UhpB, the sensor kinase. Phosphorylation of UhpA (the response regulator) by UhpB promotes transcription of the uhpT gene. Interestingly, the kinase domain of inactive UhpB can bind and sequester UhpA (Wright JS, 2000). Efficient transcription of uhpT requires both UhpA and the CRP-cAMP complex (Olekhnovich IN, 1999); UhpT at UniProtKB.
UhpT has a broad range of substrates, usually hexose phosphates, however some are poor inducers of the Uhp system. Cyclic AMP levels are low in strains growing in minimal medium supplemented with any one of the UhpT substrates, in fact among the lowest as compared to other carbon sources including glucose, provided the Uhp system is fully induced (Dumay V, 1996). Unfortunately, it remains unknown how glucose 6-phosphate transport and/or metabolism affect adenylate cyclase activity. Phosphorylated Enzyme IIAGlc, the major regulator of adenylate cyclase, is not a priori involved in glucose 6-phosphate transport and metabolism. However, one may question the phosphorylation state of Enzyme IIAGlc during glucose 6-phosphate utilization.
In 1996, it was shown that glucose 6-phosphate transport does not cause inducer exclusion. Thus it is unlikely glucose 6-phosphate transport and/or metabolism indirectly regulate adenylate cyclase by a mechanism involving dephosphorylation of Enzyme IIAGlc. In addition, it was concluded that regulation of adenylate cyclase is not related to glucose 6-phosphate metabolism but occurs during glucose 6-phosphate transport by some unknown mechanism (Dumay V, 1996)1.
In 1998, the finding that glucose 6-phosphate transport does not cause inducer exclusion was challenged by Hogema BM, 1998a2. It was proposed that Enzyme IIAGlc is dephosphorylated upon addition of glucose 6-phosphate in the culture medium. In another paper by Hogema BM, 1998b, it was further proposed, in contradiction with Dumay V, 1996, that glucose 6-phosphate metabolism is essential for controlling the phosphorylation state of Enzyme IIAGlc, the [PEP]/[pyruvate] ratio determining such phosphorylation state. Emergence of a clear picture? Not quite (Commentary 2).
Eppler T, 2002 concluded, partly in agreement with the findings by Dumay V, 1996, "it is Enzyme IIAGlc-P-dependent simulation of adenylate cyclase that is inhibited by glycerol 3-phosphate or other phosphorylated sugars, such as glucose 6-phosphate". In other words, the authors agreed with Dumay V, 1996 Enzyme IIAGlc is not dephosphorylated during glucose 6-phosphate transport and metabolism. They however proposed it was glucose 6-phosphate per se that prevented activation of adenylate cyclase by phosphorylated Enzyme IIAGlc, in contrast with Dumay V, 1996 who proposed it was glucose 6-phosphate transport that caused such activation to be prevented.
Previously, in 1973, it was shown that transport, the primary function of the PTS, is inhibited by glucose 6-phosphate (Kornberg HL, 1973). The molecular mechanism leading to this inhibition has yet to be established. However one may ponder if it is the same mechanism that prevents activation of adenylate cyclase by phosphorylated Enzyme IIAGlc during glucose 6-phosphate transport.
Because PTS transport is inhibited by glucose 6-phosphate, glucose 6-phosphate is taken up preferentially when glucose (or any other PTS-sugar) and glucose 6-phosphate are present in the culture medium. This indicates a selective preference in transport leading to uptake of substrates whose metabolism is most beneficial to Escherichia coli, at least under laboratory conditions. However, unlike glucose transport, glucose 6-phosphate transport does not cause inducer exclusion. Thus, in the presence of both glucose 6-phosphate and lactose, diauxie most likely does not occur, as inducer exclusion and a relatively low level of cAMP are both required for full manifestation of the glucose-lactose diauxie (Crasnier-Mednansky M, 2008). Nevertheless glucose 6-phosphate utilization should prevail over lactose utilization, thus leading to biphasic growth.
1 Dumay V, 1996 stated, "… the mechanism of 'catabolite repression' by hexose phosphate may be different from the one occurring with glucose" (as it does not occur upon dephosphorylation of Enzyme IIAGlc). This statement was misinterpreted by Bettenbrock K, 2007 who improperly stated Dumay V, 1996 reported that "glucose 6-phosphate did not elicit catabolite repression although cAMP levels were very low during growth with glucose 6-phosphate."
2 The authors mistakenly considered the possibility that, in wild type strains, glucose 6-phosphate was converted to glucose by a periplasmic phosphatase by referring to data by Dumay V, 1993. In fact, Dumay V, 1993 demonstrated glucose 6-phosphate is indeed converted to glucose by a periplasmic phosphatase but only in uhp mutant strains.
