Isolation of Vitamin B 12 Transport Mutants of Escherichia coli

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Abstract
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.