Selection of biocatalysts for chemical synthesis

To determine whether microbial chemosensors can be used to find new or better biocatalysts, we constructed Escherichia coli hosts that recognize the product of a biocatalytic conversion through the transcriptional activator NahR and respond by expression of a lacZ or tetA reporter gene. Equipped with a benzaldehyde dehydrogenase (XylC from Pseudomonas putida), the lacZ-based host responded to the oxidation of benzaldehyde and 2-hydroxybenzaldehyde to the corresponding benzoic acids by forming blue colonies, whereas XylC- cells did not. Similarly, the tetA-based host was able to grow under selective conditions only when equipped with XylC, enabling selection of biocatalytically active cells in inactive populations at frequencies as low as 10(-6).