Derepressed leucine transport activity in Escherichia coli

Transport activity and synthesis of binding protein for the amino acids leucine, isoleucine and valine in E. coli are coordinately controlled by the level of leucine in the growth medium. Spontaneous mutants ( dlu ) which can utilize D‐leucine as a source of L‐leucine show derepressed transport activity for the three‐branched chain amino acids. The increased transport activity is a result of an increase in the binding protein for these amino acids. Azaleucine‐resistant mutants have been isolated which have a defect in leucine transport but normal levels of the binding protein for leucine.