Cloning in Escherichia coli K-12 of a Na+-dependent transport system from a marine bacterium.

The transport of D-alanine by Escherichia coli K-12 neither requires nor is stimulated by Na+. The transport of D-alanine by the marine bacterium Alteromonas haloplanktis 214 requires Na+ specifically. Mutants of E. coli which were unable to transport D-alanine were isolated by enrichment for D-cycloserine resistance. One of the mutants was transformed with a gene bank of A. haloplanktis chromosomal DNA. Two transformants, E. coli RM1(pPM1) and E. coli RM1(pPM2) were able to transport D-alanine by a Na+-dependent mechanism. Li+ and K+ were unable to replace Na+. Both transformants contained chimeric plasmids with inserts which hybridized with A. haloplanktis but not E. coli chromosomal DNA or each other. Despite the lack of homology between the inserts, Na+-dependent D-alanine transport in the two transformants could not be distinguished either by kinetic studies or by differences in the capacity of various amino acids to compete for D-alanine uptake.