Mechanism of d -Cycloserine Action: Transport Mutants for d -Alanine, d -Cycloserine, and Glycine

The accumulation of d -alanine and the accumulation of glycine in Escherichia coli are related and appear to be separate from the transport of l -alanine. The analysis of four d -cycloserine-resistant mutants provides additional support for this conclusion. The first-step mutant from E. coli K-12 that is resistant to d -cycloserine was characterized by the loss of the high-affinity line segment of the d -alanine-glycine transport system in the Lineweaver-Burk plot. This mutation, which is linked to the met 1 locus, also resulted in the loss of the ability to transport d -cycloserine. The second-step mutation that is located 0.5 min from the first-step mutation resulted in the loss of the low-affinity line segment for the d -alanine-glycine transport system. The transport of l -alanine was decreased only 20 to 30% in each of these mutants. A multistep mutant from E. coli W that is 80-fold resistant to d -cycloserine lost >90% of the transport activity for d -alanine and glycine, whereas 75% of the transport activity for l -alanine was retained. E. coli W could utilize either d - or l -alanine as a carbon source, whereas the multistep mutant could only utilize l -alanine. Thus, a functioning transport system for d -alanine and glycine is required for both d -cycloserine action and growth on d -alanine.