Isolation of vitamin B 12 transport mutants of Escherichia coli.

Escherichia coli KBT001, a methionine-vitamin B(12) auxotroph, was found to require a minimum of 20 molecules of vitamin B(12) (CN-B(12)) per cell for aerobic growth in the absence of methionine. After mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine and penicillin selection, two kinds of B(12) transport mutant were isolated from this strain. Mutants of class I, such as KBT069, were defective in the initial rapid binding of CN-B(12) to the cell and were unable to grow in the absence of methionine even with CN-B(12) concentrations as high as 100 ng/ml. The class II mutants possessed intact initial phases of CN-B(12) uptake but were defective in the secondary energy-dependent phase. These strains were also unable to convert the CN-B(12) taken up into other cobalamins. In the absence of methionine, some of these strains (e.g., KBT103) were able to grow on media containing 1 ng of CN-B(12)/ml, whereas others (e.g., KBT041) were unable to grow with any of the CN-B(12) concentrations used. Osmotic shock treatment did not affect the initial rate of uptake of CN-B(12) but gave a substantial decrease in the secondary rate. Trace amounts of B(12)-binding macromolecules were released from the cells by the osmotic shock, but only from strains such as KBT001 and KBT041 which possessed an active initial phase of CN-B(12) uptake. These results are interpreted as being consistent with the view that the initial CN-B(12) binding site which functions in this transport system is probably bound to the cell membrane.