AbstractPhenylalanine ammonia-lyases (PALs) catalyse the non-oxidative deamination of l-phenylalanine to trans-cinnamic acid, while in the presence of high ammonia concentration the reverse reaction occurs. PALs have been intensively studied, however, their industrial applications for amino acids synthesis remained limited, mainly due to their decreased operational stability or limited substrate specificity. The application of extensive directed evolution procedures to improve their stability, activity or selectivity, is hindered by the lack of reliable activity assays allowing facile screening of PAL-activity within large-sized mutant libraries. Herein, we describe the development of an enzyme-coupled fluorescent assay applicable for PAL-activity screens at whole cell level, involving decarboxylation of trans-cinnamic acid (the product of the PAL reaction) by ferulic acid decarboxylase (FDC1) and a photochemical reaction of the produced styrene with a diaryltetrazole, that generates a detectable, fluorescent pyrazoline product. The general applicability of the fluorescent assay for PALs of different origin, as well as its versatility for the detection of tyrosine ammonia-lyase (TAL) activity have been also demonstrated. Accordingly, the developed procedure provides a facile tool for the efficient activity screens of large mutant libraries of PALs in presence of non-natural substrates of interest, being essential for the substrate-specificity modifications/tailoring of PALs through directed evolution-based protein engineering.