FIGURE 3. Steady-state protein copy number distributions in a population of cells.

a, Histogram of FDG hydrolysis rates from 48 chambers containing chloroform-treated E. coli cells shows discrete, evenly spaced peaks, with spacing equal to that measured with purified enzyme (Fig. 1f). b, Copy number distribution of -gal molecules in chambers containing a single E. coli cell, under the repressed condition (120 cells). The solid line shows a gamma distribution (equation (1)) with a = 0.16 bursts per cell cycle and b = 7.8 -gal molecules per burst. Inset: average hydrolysis rate grows linearly with the number of cells per chamber with a slope of 0.9 enzymes per cell. c, Histogram of the number of -gal molecules in chloroform-treated yeast cells (116 cells) expressing -gal from the repressed GAL1 promoter carried on a low copy number plasmid. The solid line is a gamma distribution with a = 0.2 and b = 1.7.