Characterization of a novel L-serine transport system in Escherichia coli.

A novel transport system for L-serine was found in Escherichia coli cells grown on medium containing amino acid mixture. This novel system is distinguishable from the known three transport systems for L-serine, namely, the serine-threonine system, one of the leucine-isoleucine-valine systems, and the glycine-alanine system. Uptake of L-serine via this novel system was inhibited by none of the amino acids tested, indicating that it is highly specific for L-serine. This system was induced by L-leucine, but not by L-serine. The Km for L-serine was 50 microM, and the Vmax was 23 nmol/min per mg of cell protein. Transport of L-serine via this system was strongly inhibited by KCN, an inhibitor of the respiratory chain, or by carbonyl cyanide m-chlorophenylhydrazone, an H+ conductor. Uptake of H+ was induced by L-serine influx. These results indicate that an H+-serine cotransport mechanism is operative in this novel L-serine transport system.