Transport properties of merodiploids covering the dagA locus in Escherichia coli K-12.

A membrane componenet of the dag transport system which serves for glycine, D-alanine, and D-serine is coded for by the dagA gene at minute 83 of the Escherichia coli chromosome. Merodiploid strains (dagA+/dagA+) show two to three times the transport activity for only those amino acids that are substrates of the dag transport system. The increased transport activity is a result of a two-to threefold increase in Vmax for amino acid uptake with little or no change in the Km value. The two- to threefold gene dose effect of the merodiploid strains is maintained even during carbon starvation, eliminating the possibility that a greater energy supply for transport activity may account for the effect. Since merodiploids which carry more than one copy of the dagA allele show a gene dose response for transport activity, we conclude that the membrane componenet of the dag transport system which is coded for by the dagA allele is present in limiting amounts.