<p>The Trk system is a low to medium affinity potassium uptake system, widely found in both in bacteria and archaea, where the uptake of K(+) is believed to be linked to H(+) symport [<cite idref="PUB00042655"/>]. The core Trk system consists of two proteins, the integral membrane K(+)-translocating protein TrkH (or TrkG), and the NAD-binding peripheral membrane protein TrkA [<cite idref="PUB00042656"/>]. In <taxon tax_id="562">Escherichia coli</taxon> the activity of TrkH is dependent on the ATP-binding protein SapD (also known as TrkE) which is part of the SapABCDF ABC transporter of unkown function. Not all Trk systems are dependent on SapD however - it is thought that these may utilise ATP-binding proteins from other ABC transporters [<cite idref="PUB00042657"/>].</p><p>Sequence analysis of TrkA reveals that the protein consists of 2 tandemly arrayed halves that are 22% identical, a situation that might have arisen through gene duplication. TrkA also exhibits similarity to other <taxon tax_id="562">Escherichia coli</taxon> proteins [<cite idref="PUB00001263"/>], in particular KefC, a glutathione- regulated efflux protein [<cite idref="PUB00003817"/>], and to various dehydrogenase proteins that possess NAD<sup>+</sup> binding sites.</p>