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Center for Nonlinear Studies |
How cells sense their environment using signal transduction pathways and respond to environmental changes by regulating gene expression is a key problem in systems biology. Our research focuses on the high-osmolarity glycerol (HOG) pathway, which is one of the mitogen-activated protein kinase (MAPK) pathways in Saccharomyces cerevisiae yeast cells. During the last few decades, the components and regulatory network of this pathway have been elucidated via genetic and biochemical assays performed on large populations of yeast cells. However, surprisingly little is known about the dynamics of signal transduction and its regulation of gene expression in individual cells. We have found that signal transduction dynamics and intensity in the HOG - pathway in single cell is homogeneous but the subsequent gene expression of osmosensitive genes is heterogeneous. In addition, we found that gene expression pattern is bi-modal leaving a constant fraction of cells non-responsive independent of the strength of the imposed signal. Furthermore, we found through single cell time-lapse microscopy experiments that gene expression activation after repetitive signal transduction is random and memory less. We identify a mechanism, which explains these findings by using a combination of quantitative single molecule experiments, mathematical modeling and genetics. Quantitative experiments have been performed using the single molecule RNA FISH technique to determine the absolute number of endogens mRNA transcripts following transcription activation. We analyzed the obtained mRNA distributions with single molecule based model approaches and identified a unique model class of gene activation which consists of a three state. This model class suggests being chromatin related and helped us to identify a key transcriptional regulator which modulates the RNA transcription dynamics. From the experimental and theoretical analysis of the HOG signal transduction and gene regulation pathway, we conclude that the three state model of gene activation represent three chromatin states that function as dynamic buffer in the activation of gene expression for fast environmental changes.
Host: Brian Munsky