Fig. 3.

Determining informational variation for osmosensing in budding yeast allows us to predict the probability of the different osmotic conditions experienced by yeast. (A) Hyperosmotic stress is sensed by two pathways in budding yeast, which activate the MAP kinase kinase kinases Ste11 and Ssk2/22 (22). Both these kinases activate the MAP kinase kinase Pbs2, which in turn activates the MAP kinase Hog1. Activated Hog1 translocates from the cytosol to the nucleus and initiates new gene expression. (B) Histograms of fluorescence data from a YFP reporter expressed from the promoter for STL1 and measured by Pelet et al. (19). Fluorescence levels typically increase with increasing extracellular salt: Blue corresponds to zero extracellular salt; dark green to 0.05 M salt; red to 0.1 M; cyan to 0.15 M; magenta to 0.2 M; and brown to 0.4 M. Approximately 1,000 data points were measured for each concentration (19) and are shown using 20 bins for the fluorescence level (calculated in log-space). The left inset shows the same histograms but weighted by the probability of the different salt concentrations for an input distribution that has a low informational fraction of output variance; the right inset is analogous but for an input distribution that has a high informational fraction of output variance. (C) The five probability distributions for extracellular salt that give the five highest informational fractions (each approximately equal to 0.8 because of the high degree of overlap of the fluorescence distributions for zero and 0.05 M salt). Each distribution is read horizontally. We calculated the informational fraction for all possible probability distributions of the six concentrations of extracellular salt that were chosen experimentally. The informational fraction decreases continuously from around 0.8 to zero. A uniform probability distribution of salt gives an informational fraction of approximately 0.6.