Identification of the 4‐hydroxyphenylacetate transport gene of Escherichia coli W : construction of a highly sensitive cellular biosensor

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Abstract
The mechanism of uptake of 4-hydroxyphenylacetate (4-HPA) by Escherichia coli W. was investigated. The 4-HPA uptake was induced by 4-HPA, 3-hydroxyphenylacetate (3-HPA) or phenylacetate (PA) and showed saturation kinetics with apparent K t and V max values of 25 μM and 3 nmol/min per 109 cells, respectively. Transport of 4-HPA was resistant to N,N″-dimethylcarbodiimide (DCCD), but was completely inhibited by cyanide and 4-nitrophenol, and, to a lower extent, by arsenate and azide, suggesting that energy is required for the uptake process. Competition studies showed that 4-HPA uptake was inhibited by 3-HPA or 3,4-dihydroxyphenylacetate (3,4-DHPA) but not by 2-hydroxyphenylacetate (2-HPA), l-tyrosine or other structural analogues, indicating a narrow specificity of the transport system. We have demonstrated, using two experimental approaches, that the hpaX gene of the 4-HPA catabolic cluster, which encodes a protein of the superfamily of transmembrane facilitators, is responsible for 4-HPA transport. Aside from the aromatic amino acid transport systems, hpaX is the first transport gene for an aromatic compound of enteric bacteria that has been characterized. A highly sensitive cellular biosensor has been constructed by coupling the 4-HPA transport system to a regulatory circuit that controls the production of β-galactosidase. This biosensor has allowed us to demonstrate that the transport system performs efficiently at very low external concentrations of 4-HPA, similar to levels that would be expected to occur in natural environments.

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