Utilization and Transport of Hexoses by Mutant Strains of Salmonella typhimurium Lacking Enzyme I of the Phosphoenolpyruvate-dependent Phosphotransferase System

Mutant strains of Salmonella typhimurium that lack Enzyme I of the phosphoenolpyruvate-dependent phosphotransferase system cannot take up several sugars either by group translocation or by facilitated diffusion. A second mutation, which resulted in a 25-fold enhancement of the ATP-dependent manno(fructo)kinase activity, partially restored the ability of the cells to take up the sugars that this enzyme phosphorylates. It is suggested that phosphorylation of these sugars is required for their release from a membrane carrier into the cytoplasm. Preliminary evidence indicates that the mutation affecting the constitutively synthesized manno(fructo)kinase results in increased levels of the protein, rather than in a structural change affecting catalytic activity. The mutation may represent one of few known examples suggesting that the levels of constitutive enzymes are under genetic regulation.