<p>Among the different families of transporter only two occur ubiquitously in all classifications of organisms. These are the ATP-Binding Cassette (ABC) superfamily and the Major Facilitator Superfamily (MFS). The MFS transporters are single-polypeptide secondary carriers capable only of transporting small solutes in response to chemiosmotic ion gradients [<cite idref="PUB00007278"/>, <cite idref="PUB00007279"/>].</p><p>This entry represents a group of MFS proteins encoded in a number of proteobacterial genomes, but only in the context of having genes for 4-hydroxyphenylacetate (4-HPA) degradation. The protein is characterised as 4-HPA permease in <taxon tax_id="562">Escherichia coli</taxon>, where 3-HPA and 3,4-dihydroxyphenylacetate, but not other substrate analogues, were shown to competitively inhibit 4-HPA transport, indicating a narrow substrate specificity [<cite idref="PUB00017754"/>].</p> 4-hydroxyphenylacetate permease