Stochastic Gene Expression in a Single Cell

HDAC1 were also bound to the same region (Fig. 5E), which is consistent with the recruitment of at least two corepressor complexes through Dachs. To test whether Six6 could directly regulate p27Kip1 expression in a biological context, we performed ChIP experiments using microdis-sected e13.5 wild-type retinas, showing that both Six6 and Dach2 were indeed recruited to the putative SE sites of the p27Kip1 promoter (Fig. 5F), which is consistent with the correlation between Six6 high expression and p27Kip1 low expression at that developmental time (13). Despite the strong expression of Sno in the developing retina and its homology with Dach2, Sno was not present on the p27Kip1 promoter (Fig. 5F), which is consistent with our finding of no detectable functional interactions between Six6 and Sno in transient transfection, two-hybrid, and microinjection studies (Fig. 4D) (13). We therefore conclude that Six6/Dach complex binds directly to the p27Kip1 promoter and represses its transcriptional activity in vivo, together with regulation of p19Ink4d and p57Kip2, to regulate proliferation. The Six6/CKI regulatory network likely serves as a molecular strategy for Six6-dependent regulation of the proper expansion of retinal and pituitary precursor cell populations. The strong coexpression of another highly related Six gene, Six3, during retinal development could partially compensate for the loss of Six6 (5). Six6/Dach repressive function in eye development is in contrast to the activation roles shown for Six1/Eya2 in muscle development (11), identifying a unique role of Six6 in terms of regulating downstream genes by interacting with specific partners. Together, these findings provide an organ-specific strategy for the expansion of precursor cell populations during development , a strategy that is likely used in other organ systems. and manuscript preparation; and H. Taylor for animal care. We are very grateful to R. Clonal populations of cells exhibit substantial phenotypic variation. Such het-erogeneity can be essential for many biological processes and is conjectured to arise from stochasticity, or noise, in gene expression. We constructed strains of Escherichia coli that enable detection of noise and discrimination between the two mechanisms by which it is generated. Both stochasticity inherent in the biochemical process of gene expression (intrinsic noise) and fluctuations in other cellular components (extrinsic noise) contribute substantially to overall variation. Transcription rate, regulatory dynamics, and genetic factors control the amplitude of noise. These results establish a quantitative foundation for modeling noise in genetic networks and reveal how low intracellular copy numbers of molecules can fundamentally …