Dynamic single cell analysis of a MAPK signaling cascade and its impact on transcriptional output

We aim to understand how the signal transduction from the cascade impacts the transcriptional activation of stress-responsive genes. To this end the yeast \textit{Saccharomyces cerevisiae} is used to study the dynamics of the MAPK Hog1 during an hyper-osmotic stress response. It is possible to quantify the activity of Hog1 by monitoring its relocation upon induction by osmotic stress as it accumulates in the nucleus. Downstream osmo-stress inducible promoters such as p\textit{STL1}, p\textit{HSP12}, p\textit{GPD1} are functionalized with the PP7 system to label nascent mRNAs. This system comprises 24 stem loop repeats which are bound by a labelled phage coat protein. As soon as the mRNA is transcribed, these loops form and a fluorescent signal which accumulates at the active locus can be observed and quantified. From the signals extracted out of these images key parameters of the promoter output are characterised such as peak transcriptional activity, duration and total output. These parameters are correlated to the MAPK dynamics in order to study the impact of MAPK activity on transcritonal response. We report that the MAPK activity pattern we measure is not a predictor of the promoter expression . We investigate whether the global transcriptional capability of the cell conditions the level of transcriptional response and heterogeneity in single cell behaviour. In parallel we are building a mathematical model of MAPK driven transcription using time dependent rates based on experimental measurements and estimates.