Sugar transport by the bacterial phosphotransferase system. Isolation and characterization of a phosphocarrier protein HPr from wild type and mutants of Salmonella typhimurium.

HPr, the histidine-containing phosphocarrier protein of the phosphotransferase system, has been isolated and purified from wild type Salmonella typhimurium and from two mutants of this organism. Comparison of the sequences of these three forms of HPr indicates that the two mutants contain substitutions at residue 4 in the polypeptide chain; in place of glutamine in the wild type, one mutant (SB3899) contained serine and the other (SB3093) contained lysine. The substitution of lysine for glutamine resulted in increased positive charge of the molecule which is reflected in an expected decreased mobility on polyacrylamide gel electrophoresis and its behavior on isoelectric focusing. However, these changes had no effect on phosphocarrier activity in the phoshoenolpyruvate:glycose phosphotransferase system as measured by the kinetics of the interaction on the mutant HPr proteins with both Enzyme I and Enzyme II. These results have important implications for potential chemical modification of HPr. The HPr from wild type cells was crystallized.