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Abstract
Preferential sugar utilization is a widespread phenomenon in biological systems. Glucose is usually the most preferred carbon source in various organisms, especially in bacteria where it is taken up via the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The currently proposed model for glucose preference over non-PTS sugars in enteric bacteria including E. coli is strictly dependent on the phosphorylation state of the glucose-specific PTS component, enzyme IIAGlc (EIIAGlc). However, the mechanism of the preference among PTS sugars is largely unknown in Gram-negative bacteria. Here, we show that glucose preference over another PTS sugar, mannitol, is absolutely dependent on the general PTS component HPr, but not on EIIAGlc, in E. coli. Dephosphorylated HPr accumulates during the transport of glucose and interacts with the mannitol operon regulator, MtlR, to augment its repressor activity. This interaction blocks the inductive effect of mannitol on the mannitol operon expression and results in the inhibition of mannitol utilization.
Highlights
The molecular mechanism for inhibition of expression and/or activity of proteins required for the transport and metabolism of less-preferred carbon sources in the presence of a preferred carbon source has been most extensively studied in E. coli, and the best-studied example of this Carbon catabolite repression (CCR) is the glucose-lactose diauxie, which was first observed by Jacques Monod in 19426
To verify that EIIAGlc is not implicated in the preference of glucose over the phosphotransferase system (PTS) sugar mannitol, we performed a sugar preference test in an E. coli mutant deleted of the crr gene encoding EIIAGlc (Fig. 1)
It is known that glucose can be transported either by the glucose PTS or by the mannose PTS in E. coli and other enteric bacteria, the mannose PTS can be less efficient than the glucose PTS in uptake and phosphorylation of glucose in E. coli[7,11]
Summary
The molecular mechanism for inhibition of expression and/or activity of proteins required for the transport and metabolism of less-preferred carbon sources in the presence of a preferred carbon source has been most extensively studied in E. coli, and the best-studied example of this CCR is the glucose-lactose diauxie, which was first observed by Jacques Monod in 19426. Because dephospho-EIIAGlc can interact with and inhibit several non-PTS permeases including the lactose permease[7], transport and/or metabolism of less preferred carbon sources is prevented in the presence of glucose. No diauxie was observed when cells were provided simultaneously with glucose and mannitol, the activity of the mannitol transport system was shown to be inhibited in the presence of glucose in E. coli[10] Such hierarchical utilizations among PTS sugars have been observed in various Gram-negative and in Gram-positive bacteria as well[7]. In this study, the molecular mechanism of the preference between glucose and mannitol in E. coli was investigated
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