Regulation of transcription dynamics in single cells

In mammalian cells, the MAPK pathways regulate vital processes such as differentiation, growth or death in response to a wide array of stimuli, like hormones or stresses. To achieve this, they interpret extracellular cues and reprogram the cell transiently or on longterm, through the transcriptional regulation of target genes. Understand the regulation of the MAPK signaling and resulting transcription is of high importance because of the involvement of mis-regulation of these pathways in many diseases (cancer, diabetes, cardiovascular diseases). As these signaling cascades are conserved among various organisms, we study them in the budding yeast Saccharomyces cerevisiae. This widely used model organism possesses its own MAPK network, and is easy to genetically modify. The transcription is a crucial process in cellular life, as it drives the production of proteins involved in all possible aspects of life. As such, it is highly regulated through complex combination of factors. Moreover, the transcription is a dynamic process that can occur either continuously or in a bursty manner. These bursts can be variable in their frequencies and duration, leading to various mRNA productions within cells from a clonal population. Hence, the transcription regulation needs to be studied at the single cell level and in real-time. To this purpose, we developed a new kind of gene expression reporters able to quantify in real-time and at the single cell level the expression arising from a promoter, that we named dPSTR. Using this system, we measured the extremely fast and transient gene expression resulting from hyper osmotic stress, and observed a difference on the sub-minute timescale in gene induction. Then, we used the dPSTR to quantify gene expression triggered by the mating signaling. We found that despite a signaling activity occurring minutes after addition of exogenous pheromone, resulting gene expression occurs on different timescales. Finally, by means of genetic analysis and modification of endogenous promoters, we defined some rules governing the expression kinetics of mating-induced genes