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Plasticity in substrate acceptance is a well-characterised phenomenon for disaccharide transporters. Sucrose, a non-reducing disaccharide, is usually metabolised  via  either the permease-mediated chromosomally-encoded sucrose catabolism ( csc ) regulon or the sucrose phosphotransferase system (PTS).  E. coli  W is a fast-growing strain which efficiently utilises sucrose at concentrations above 1%  via  the  csc  regulon. To examine if sucrose could be metabolised  via  other routes, a library of transposon mutants was generated and screened on 0.2% sucrose. One mutant identified from this library had an insertion in the repressor for the regulon controlling catabolism of the disaccharide trehalose ( treR ). A series of mutants was constructed to elucidate the mechanism of sucrose utilization in the  treR  insertion strain. Analysis of these mutants provided evidence that deletion of TreR enables uptake of sucrose  via  TreB, an enzyme II protein required for PTS-mediated uptake of trehalose. Once inside the cell, this sucrose is not processed by the TreC hydrolase, nor is it sufficient for growth of the strain. QRT-PCR analysis showed that levels of  cscA  (invertase) transcript increased in the WΔ treR  mutant relative to the wild-type strain when grown under low sucrose conditions. This result suggests that the intracellular sucrose provided by TreB can facilitate de-repression of the  csc  regulon, leading to increased gene expression, sucrose uptake and sucrose utilization in the  treR  mutant.

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The Trehalose Phosphotransferase System (PTS) in E. coli W Can Transport Low Levels of Sucrose that Are Sufficient to Facilitate Induction of the csc Sucrose Catabolism Operon