The ptsHI-crr operon in E. coli contains three genes which encode proteins that are part of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PEP group translocation or sugar PTS). This is an active transport system used to bring sugars such as glucose and mannose into the cell. The operon can be activated by the cAMP-activated global transcriptional regulator CRP (CAP), which binds upstream of the promoter region and interacts with RNA polymerase, activating transcription of the operon. The operon can also be inhibited in several locations by various proteins. The catabolite repressor/activator protein Cra can bind to the promoter region, and depending on the binding of CRP, can either activate the operon's transcription if CRP is not bound, or can inhibit it if CRP is bound. Protein mlc is a repressor that can bind to the promoter region of the operon, repressing transcription. It is involved in the repression and regulation of other proteins involved in the sugar PTS. Finally, the N-acetylglucosamine repressor can bind to the promoter region after being activated by the binding of N-acetyl-D-glucosamine 6-phosphate, allowing it to inhibit transcription of the operon. The first gene in the operon, ptsH, encodes the phosphocarrier protein HPr, which is a general carrier protein in the sugar PTS that is not specific to the sugar being transported by the system. It takes a phosphoryl group from phosphoenolpyruvate (PEP) and transfers it to the EIIA domain of the carrier. The second gene, ptsI, encodes phosphoenolpyruvate-protein phosphotransferase, another component of the sugar PTS that is not specific to the sugar. This enzyme is responsible for transferring the phosphoryl group from PEP to the protein HPr. The final gene in the operon, crr, encodes theglucose-specific phosphotransferase enzyme IIA component, which combines with the PTS system N-acetylmuramic acid-specific EIIBC component to form the Enzyme E II protein. First, EIIA takes the phosphoryl group from HPr and transfers it to EIIB. Then EIIB transfers it across the cell membrane to EIIC via glucose, forming glucose-6-phosphate, which does not exit the cell naturally, allowing more glucose to be pumped into the cell forming a gradient.

Operon: Sugar Uptake References