Figure 2.

Measuring intrinsic and extrinsic noise in E. coli. Intrinsic noise can be differentiated from extrinsic noise by measuring the activity of two allelic GFP variants, YFP (shown in red) and CFP (shown in green), under control of the same promoter. (A) When intrinsic noise is absent within a single cell, the fluorescence intensity of YFP and CFP should be identical (represented by yellow cells). Extrinsic noise will cause different cells within the population to exhibit variations in overall fluorescent intensity. (B) Intrinsic noise within a single cell will cause the fluorescence intensity of YFP and CFP to differ, resulting in some green and some red tinted cells. Reproduced from Elowitz et al. (2002). Reprinted with permission from AAAS.