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. Author manuscript; available in PMC: 2016 Sep 11.
Published in final edited form as: Science. 2016 Mar 10;351(6278):1218–1222. doi: 10.1126/science.aad0635

Fig. 2. Measuring the number of CI molecules in individual cells.

Fig. 2

(A) CI proteins were labeled using antibodies to CI and fluorescently-labeled secondary antibodies (left). Under the microscope, lysogenic E. coli cells exhibited a strong CI signal (right) whereas non-lysogens showed a weak background signal (center). (B) Method #1 for measuring the number of CI proteins per cell. The typical fluorescence of a single CI dimer was obtained from the spot intensity distribution in lysogenic cells (green, N = 23631 spots), distinguishable from that of the negative sample (black, N = 1764 spots). (C) Method #2 for measuring the number of CI proteins per cell. The variance versus the mean of pixel intensity in individual cells (gray, N = 324) was fitted to a linear function (green). The slope of this line was used to estimate the fluorescent intensity of a single CI dimer. (D) The estimated number of CI molecules in a lysogen, obtained using the two single-cell methods (green, mean ± SEM from 6 experiments, 327 to 704 cells each). Also shown is the value reported in the literature (gray, mean ± SD from three studies (19–21)). (E) The distribution of CI copy number in lysogenic cells (green; N = 560 cells). The data is described well by a gamma distribution (black).