| Abstrakt: |
The products of three genes are involved in cyanocobalamin (B12) uptake in Escherichia coli. btuB(formerly bfe), located at min 88 on the Escherichia colilinkage map, codes for a protein component of the outer membrane which serves as receptor for B12, the E colicins, and bacteriophage BF23. Four phenotypic classes of mutants varying in response to these agents were found to carry mutations that, based on complementation and reversion analyses, reside in the single btuBcistron. In one mutant class, ligand binding to the receptor appeared to be normal, but subsequent B12uptake was defective. The level of receptor and rate of uptake were responsive to btuBgene dosage. Previous studies showed that the tonBproduct was necessary for energy-dependent B12uptake but not for its binding. Other than those in tonB, no mutations that conferred insensitivity to group B colicins affected B12utilization. The requirement for the btuBand tonBproducts could be bypassed by elevated levels of B12(>1 μM) or by mutations compromising the integrity of the outer membrane as a permeability barrier. Utilization of elevated B12concentrations in strains lacking the btuB-tonBuptake system was dependent on the function of the btuCproduct. This gene was located at 37.7 min on the linkage map, with the order pps-btuC-pheS. Strains altered in btuCbut with an intact btuB-tonBsystem were only slightly impaired in B12utilization, being defective in its accumulation. This defect was manifested as inability to retain B12, such that intracellular label was almost completely lost by exchange or efflux. It is proposed that btuCencodes a transport system for B12in the periplasm. |
