Wild-type Escherichia coli possesses an inducible permeation system which catalyzes facilitated diffusion of glycerol into the cell. A spectrophotometric method can be used to assess the presence of this mechanism. The structural gene for the facilitator (glpF) and the structural gene for glycerol kinase (glpK) apparently belong to a single operon. The glpF+ allele permits effective glycerol utilization by the cells, and, at millimolar concentrations of glycerol, cells carrying the glpF+ allele grow much faster than glpF genotypes. Although the glycerol-scavenging power of the cell depends both on the facilitated entry of the substrate and its subsequent trapping by an adenosine triphosphate-dependent phosphorylation, the two gene products, the facilitator and kinase, function independently. Wild-type Shigella flexneri appears to be glpK+ but glpF. This organism grows slowly in media at low concentrations of glycerol. When the glpF+ and glpK+ alleles of E. coli are inserted into the S. flexneri genome by transduction, the hybrid strain grows rapidly in low glycerol medium. Vice versa, when the gIpF and glpK+ alleles of S. flexneri are incorporated into E. coli, the hybrid strain grows slowly in low glycerol medium. A facilitated diffusion system for the entry of glycerol has been found in Escherichia coli. This system is inducible and is expressed under the control of the glpR gene (14). Figure
A glycerol-specific phenotypic revertant isolated from a mutant of Escherichia coli missing enzyme I of the phosphoenolpyruvate phosphotransferase system was studied. This revertant is capable of producing higher levels of glycerol kinase and the protein mediating the facilitated diffusion of glycerol (facilitator) than wildtype cells. The kinase of the revertant is indistinguishable from the wild-type enzyme with respect to its sensitivity to feedback inhibition by fructose-1, 6-diphosphate, its pH optimum, and its turnover number. The synthesis of glycerol kinase in strains bearing the suppressor locus is resistant to catabolite repression. The suppressor mutation mapped at the known glpK locus. Thus, it is suggested that the mutation occurred in the promoter of the operon specifying the kinase and the facilitator.
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