Abstract: The present invention relates to the use of transcriptional activators from prokaryotic organisms for use in eukaryotic cells, such as yeast as sensors of intracellular and extracellular accumulation of a ligand or metabolite specifically activating this transcriptional activator in a eukaryot, such as yeast cell, such as a cell engineered to produce this ligand. The transcriptional activator controls a promoter upstream of one or more gene, which may include e.g. a reporter gene that may be a fluorescence marker, such as luciferase, green fluorescent protein or a gnee encoding antibiotic resistance.