Importance of facilitated diffusion for effective utilization of glycerol by Escherichia coli

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 glpF and glpK(+) alleles of S. flexneri are incorporated into E. coli, the hybrid strain grows slowly in low glycerol medium.