Fig. 5.

Three strategies for changing the specificity of chemoreceptors. The rational design strategy starts with the full understanding of receptor structures, mechanisms, and functions. Residues that may bind to a novel chemoeffector are predicted rationally. The variant genes are obtained by site-directed mutagenesis and then expressed in E. coli [40, 142]. The directed evolution strategy needs mutant gene libraries that can be prepared by random mutagenesis or gene fragment recombination [141–143]. Libraries of E. coli variants are obtained after gene transformation and expression. Making hybrid chemoreceptors is another strategy to change the ligand specificity. Two receptor genes can be fused together according to the sequence conservation [141]. The non-native periplasmic sensing domains in hybrid receptors enable E. coli to sense novel ligands. The methods for selecting desirable E. coli variants that can sense novel chemoeffectors are various, such as capillary assay [60], agar plate assay [143], microfluidic assay [40], and FRET assay [68]. The three strategies can be successively used in combination to optimize the desired function