Abstract #24

Two “redundant” inhibitors of the MAPK-responsive transcription factor Ste12 differentially modulate noise during yeast pheromone signaling. Emma McCullagh, Anupama Seshan, Hana El-Samad, Hiten Madhani. Biochemistry and Biophysics, UCSF, San Francisco, CA.
   Variability or “noise” in the transcriptional responses of individual isogenic cells is a ubiquitous, yet incompletely understood, phenomenon. Studies in bacteria and yeast indicate that the process of gene expression itself can be intrinsically noisy. This intrinsic gene expression noise is thought to occur because of the stochastic impacts of small numbers of molecules and the synthesis and turnover kinetics of mRNAs and proteins. There is also increasing evidence that the kinetic steps preceding and leading to gene activation may modulate noise. The pheromone response MAPK pathway provides a tractable model system for studying the biology of noise during eukaryotic signal transduction and gene activation. The target transcription factor, Ste12, binds to a large number of genes to control both cell type-specific and pheromone-responsive transcription. Ste12 is regulated by two redundant inhibitors, Dig1 and Dig2 (also known as Rst1 and Rst2), which bind to the activation and DNA-binding domains of Ste12, respectively. To test the hypothesis that there exist pathway-specific factors that modulate the noise response of promoters, we measured total noise from three different pheromone-responsive promoters (MFA1, AGA1, and FUS1) and a non-pheromone pathway reporter (LYS1) using promoter-fluorescent protein gene fusions. Our results confirm that Dig1 and Dig2 act redundantly to repress Ste12-dependent transcription. Strikingly, we found that Dig1, but not Dig2, has a role in suppressing noise from Ste12-dependent promoters: cells lacking Dig1 display significant increases in cell-to-cell variability in their outputs. Because Dig1 binds to the activation domain of Ste12, one possible explanation for these results is that Dig1 limits the size of transcriptional bursts that occur when Ste12 binds to DNA. The results of a combined experimental and mathematical modeling analysis of the mechanism and consequences of noise suppression by Dig1 will be presented.

Return to YGM 2006 Home at SGD