Transport of vitamin B12 in Escherichia coli: common receptor system for vitamin B12 and bacteriophage BF23 on the outer membrane of the cell envelope.

We showed previously that the outer membrane of the Escherichia coli cell envelope normally contains about 200 to 250 B12 receptors, and that these receptors function both in B12 transport and as receptors for the E colicins. This paper shows that this receptor system is also shared with bacteriophage BF23. A strong positive correlation was observed between the number of B12 receptors per cell and the rate of adsorption of BF23. Cells from mutant strains that lacked B12 receptors did not adsorb BF23 particles. The rate of adsorption of BF23 to cells of a merodiploid strain (RK4151), with about 550 B12 receptors per cell, was approximately double that to cells of a normal, haploid strain. The adsorption of BF23 to hole cells, cell envelopes, outer membrane particles, and solubilized outer membranes was inhibited by vitamin B12, with 50% inhibition at B12 concentrations in the range of 0.5 to 2.0 nM. These values are close to the observed KS for B12 binding to the B12 receptors. Vitamin B12 concentrations as high as 100 nM did not inhibit adsorption of bacteriophages T5, T6, and lambdacI to cells of sensitive strains of E. coli. Bacteriophage BF23 inhibited B12 transport by whole cells and was shown to be a competitive inhibitor of B12 binding to isolated cell envelope particles. The B12/BF23 receptors from E. coli strains KBT069 (btuB69) and RK4104 (btuB69) were fully active, but the number per cell was reduced to an average value of about 0.5.