Abstract

A protein, exhibiting a high similarity to the major serine transporter of Escherichia coli, SstT, was found in Haemophilus influenzae Rd. A Na+-stimulated serine transport activity was also detected in the cells. The gene (sstT) for the Na^+/serine symporter from the chromosome of H. influenzae was cloned, and the properties of the transporter investigated. The serine transport activity was stimulated by Na^+. The uptake of Na^+ was elicited by the addition of serine or threonine into the cells, supporting the idea that these amino acids are transported by a mechanism of Na^+/substrate symport. No uptake of H^+ was elicited by the influx of serine. The serine transport via the SstT of H. influenzae was inhibited by excess threonine, which was used as another substrate. The K_m and the V_max values for the serine transport were 2.5 mM and 14 nmol/min/mg protein, respectively.

• Keywords: Haemophilus influenzae Rd; Na^+/serine-threonine symporter