Repression of synthesis of the vitamin B12 receptor in Escherichia coli

Abstract

Growth of Escherichia coli K-12 strains in the presence of the vitamin cyanocobalamin (B12) resulted in an 80 to 90% reduction in B12 uptake activity of washed cells. Coincident with the decline in uptake activity was the depression of B12-binding activity in energy-poisoned cells, suggesting that growth in B12 resulted in the repression of synthesis of the B12 receptor protein in the outer membrane. Growth in the presence of B12 led to marked reduction in sensitivity to the E colicins, whose adsorption to cells requires the B12 receptor, and to a decrease in the amount of a band on electropherograms of outer membrane proteins. That polypeptide was also missing from mutants altered at btuB, the locus encoding the B12 receptor. Addition of B12 to growing cultures resulted in the exponential decline in specific activity of B12 uptake, as expected for dilution of functional receptors by further growth. Repression of receptor synthesis appears to be regulated by the level of intracellular, rather than extracellular, B12 and is separate from the regulation of the methionine biosynthetic pathway. Mutants altered in btuC, which are defective in accumulation and retention of B12, exhibit a much lower degree of repressibility.