| Abstrakt: |
A promoter-like mutation, ptsP160, has been identified which drastically reduces expression of the genes specifying two proteins, HPr and enzyme I, of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in Salmonella typhimurium. This mutation lies between trzA, a gene specifying susceptibility to 1,2,4-triazole, and ptsH, the structural gene for HPr. It leads to a loss of active transport of those sugars that require the PTS for entry into the cell. Pseudorevertants of strains carrying this promoter-like mutation have additional lesions very closely linked to ptsP160 by transduction analysis and are noninducible for HPr and enzyme I above a basal level. Presumably, strains carrying ptsP160 are defective in the normal induction mechanism for HPr and enzyme I, and the pseudorevertants derived from them result from second-site initiation signals within or near this promoter-like element. The induction of HPr and enzyme I above their noninduced levels apparently is not required for transport of at least one PTS sugar, methyl alpha-d-glucopyranoside, since this sugar is taken up by the pseudorevertants at the same rate as by the wild type. The existence of a promoter-like element governing the coordinate inducibility of both HPr and enzyme I suggests that ptsH and ptsI constitute an operon. Wild-type levels of a sugar-specific PTS protein, factor III, are synthesized in response to the crr(+) gene in both a ptsP160 strain and its pseudorevertants; this suggests that the crr(+) gene has its own promoter distinct from ptsP. |
