The responses to pheromone and sorbitol in the presence of different Fus3 and Hog1 hybrid proteins. (a) The hybrids of Fus3 and Hog1 are shown on the left. Capital letters ABCDEF (black) each represent a segment of Fus3, while the lower-case letters abcdef (red) each represent a segment of Hog1. The relative responses to pheromone and sorbitol were measured using a FUS1 promoter-driven reporter gene to detect Fus3 activity (horizontal blue bars). Plasmids bearing the hybrid genes were introduced into cells deleted for endogenous Fus3 (fus3Δ) and the MAPK Kss1 (kss1Δ), an alternative target for Ste5 activation. Mating activity is scored from +++ to - (none). The lower panel in (a) shows the crossover response in which sorbitol activates the FUS1-driven reporter gene when there is high-copy expression of the ABcdEF hybrid. The Δ symbol indicates that the response was maintained in a Ste7-deleted background. (b) The relative responses to pheromone and sorbitol were measured using a STL1 promoter-driven reporter gene to detect Hog1 activity (represented by blue bars). Relative efficiency of growth on sorbitol is scored from +++ to -. Data in (a) and (b) are from [6]. (c) Model modified from [6] depicting the ability of different sequence patches in Fus3/Hog1 hybrids to regulate the pheromone and osmolyte activation of hybrid MAPKs.