The histidine-containing phosphocarrier protein (HPr) is a central component of the phosphoenolpyruvate: sugar phosphotransferase system that transports carbohydrates across the cell membrane of bacteria. A typical phosphotransfer sequence is phosphoenolpyruvate-->enzyme I-->HPr-->enzyme II/IIIsugar-->sugar. This is thermodynamically favourable owing to the participation of the high-energy phosphoenolpyruvate. We report here the structure of HPr from Streptococcus faecalis determined at 1.6 A resolution. Remarkable disallowed Ramachandran torsion angles at the active centre, revealed by the X-ray structure, demonstrate a unique example of torsion-angle strain that is probably directly involved in protein function. During phosphorylation, the active-centre torsion-angle strain should facilitate the phosphotransfer reaction by lowering the activation-energy barrier. A recently reported Bacillus subtilis HPr structure, which represents the phosphorylated state of HPr with no torsion-angle strain, provides direct evidence supporting our hypothesis that torsion-angle strain plays a direct part in the function of HPr. An HPr phosphotransfer cycling mechanism is proposed, based primarily on the structures of HPr and other phosphotransferase system proteins.