ISOLATION AND CHARACTERIZATION OF AN ESCHERICHIA-COLI MUTANT LACKING THE MAJOR SERINE TRANSPORTER, AND CLONING OF A SERINE TRANSPORTER GENE

L-Serine as well as L-valine inhibits the growth of Escherichia coli c ells, and L-isoleucine releases this growth inhibition. We isolated an E. coli mutant (designated as WAT9) that was able to grow on lactate (or glucose) as a carbon source even in the presence of L-serine, the parent not being able to, Cells of WAT9 were not able to grow on L-ser ine as a carbon source even if L-isoleucine was present in the culture medium, while the parental cells grew. This mutant was shown to lack the principal L-serine transporter in E. coli, the Na+/ serine symport er. This mutant is useful for analysis of the role(s) of the Na+/serin e symporter in cell physiology and as a host for the cloning of L-seri ne transporter gene(s). In fact, we cloned a gene encoding a serine tr ansporter from chromosomal DNA of E. coli using WAT9 as the host. The gene enabled the mutant cells to grow on L-serine, Transport activity for L-serine was restored in the mutant cells harboring a plasmid carr ying the gene, We partially sequenced the gene and found that it was t he tdcC gene, We showed that TdcC is an H+/serine symporter.