What are the fundamental Molecular mechanisms that allow a single cell to choose between different fates?

Noisy expression of GFP reporter from iso-clonal cell line

Noisy expression of GFP reporter from iso-clonal cell line

At the single-cell level, stochastic fluctuations in biochemical processes are inescapable due to limited numbers of molecular reactants.  These fluctuations generate substantial variability in gene expression, i.e. ‘noise’, between genetically identical cells (example right) and can enable probabilistic bet-hedging decisions.

Our studies found that HIV’s long-terminal repeat (LTR) promoter, the virus’s only promoter, is exceptionally noisy (Singh et al., 2010), exhibiting large, infrequent ‘bursts’ of transcription (Dar et al., 2012) that are the dominant source of noise (Singh et al., 2012).  LTR noise is further amplified by positive feedback from HIV’s Tat protein transactivating the LTR (Weinberger and Shenk, 2007).  Overall, the design of the HIV circuit (a bursty promoter coupled with positive-feedback circuitry to amplify stochastic fluctuations) appears to be optimized to act as a probabilistic switch allowing the virus to randomly choose between latency and active viral replication.

Our current questions explore the molecular mechanisms that enable cells to enhance or suppress (i.e. tune) noise in diverse biological systems.

We use a coupled computational-experimental approach based on time-lapse fluorescent imaging of live single cells together with mathematical modeling.  Models allow us to predict which biochemical and genetic perturbations have the greatest impact upon a circuit's output.