QUANTITATION OF TRANSCRIPTION AND CLONAL SELECTION OF SINGLE LIVING CELLS WITH BETA-LACTAMASE AS REPORTER

Gene expression was visualized in single living mammalian cells with b eta-lactamase as a reporter that hydrolyzes a substrate loaded intrace llularly as a membrane-permeant ester. Each enzyme molecule changed th e fluorescence of many substrate molecules from green to blue by disru pting resonance energy transfer. This wavelength shift was detectable by eye or color film in individual cells containing less than 100 beta -lactamase molecules. The robust change in emission ratio reveals quan titative heterogeneity in real-time gene expression, enables clonal se lection by flow cytometry, and forms a basis for high-throughput scree ning of pharmaceutical candidate drugs in living mammalian cells.