FIGURE 2. Quantitative real-time measurement of individual protein expression events in live E. coli cells.

-gal is under the control of a repressed lac promoter. a, Trace of a chamber containing dividing cells shows abrupt changes in hydrolysis rates (arrows on black curve). An empty chamber shows a constant background (red curve). b, Discrete jumps in -gal number are due to burst-like production of proteins. The number of -gal molecules is calculated by taking the time derivative of the traces in a and compensating for fluorescein photobleaching (Supplementary Information). c, Histogram of copy number of -gal molecules per burst. The distribution is well-fitted with an exponential function (black line), with an average of five proteins per burst, and is a consequence of exponential cellular lifetime of the mRNA.