Riboswitch inducible gene expression

An intronic, self-splicing riboswitch is configured for enzyme-product specificity by introducing an appropriate aptamer. This then provides a sensing-expression construct, whereby the presence of an enzyme product in the cell triggers self-splicing of the intron sequence to restore the reading frame of the reporter gene and as such to drive expression of the gene product. The sensing construct expresses a protein which marks the cell or permits its growth or survival in or on an otherwise selective media. In this way, introduction or the presence of such product sensing-reporter constructs in cells can be harnessed to provide a multi-parallel rapid screening of cells or libraries for desirable enzyme variants.