Properties of a Na+-coupled serine-threonine transport system in Escherichia coli

Based on the following experimental results we conclude that the serine-threonine transport system in Escherichia coli is a Na⁺-coupled cotransport system. (1) Addition of serine to cell suspensions induced H⁺ efflux in the presence of Na⁺. (2) Addition of serine to cell suspensions induced Na⁺ uptake by cells. (3) Imposition of an artificial electrochemical potential of Na⁺ in starved cells induced serine uptake. Some of these phenomena were observed when threonine was added instead of serine or inhibited when cells were preincubated with threonine. The Na⁺/serine (threonine) cotransport system was considerably repressed when cells were grown on a mixture of amino acids. Serine transport in cells grown in the absence of amino acids mixture was stimulated by Na⁺. The half maximum concentration of Na⁺ was 21 μM. Sodium ion increased the Vmax role=presentation style=font-size: 90%; display: inline-block; position: relative;>$\text{V}{}_{\max }$$\text{V}{}_{\max }$ of serine transport without affecting the Km role=presentation style=font-size: 90%; display: inline-block; position: relative;>$\text{K}{}_{\mathrm{m}}$$\text{K}{}_{\mathrm{m}}$.