The mechanism by which shared signaling components faithfully transduce distinct signals to elicit different outcomes is an outstanding question in eukaryotic cell signaling. Our long term objective is a molecular understanding of this ubiquitous phenomenon. In the budding yeast Saccharomyces cerevisiae, the mating response and the filamentous growth signaling pathways share seven components, yet respond to different signals and activate distinct genes. The use of common components by these two pathways offers a powerful model system for dissecting signaling specificity. Our work has revealed that the two pathways utilize distinct MAP kinases (Fus3 for mating, versus Kss1 for filamentation) and distinct combinations of transcription factors (Ste12-Mcm1 and Ste12-Ste12 for mating versus Ste12-Tec1 for filamentation). Shared components in addition to Ste12 include three protein kinases (Ste20, Ste11, Ste7) that act in a cascade upstream of the MAP kinases. We seek to understand three aspects of how the two distinct signals are faithfully transmitted: how an activated receptor stimulates only the correct MAPK; how pathway-specific MAPKs maintain specificity; and how the shared transcription factor subunit, Ste12, induces genes appropriate to the input signal. To understand the bases for signaling specificity, we have the following specific aims: (1) Uncover the mechanisms by which distinct inputs activate the appropriate MAPK through the shared upstream MAPK cascade components Cdc42-Ste20, Ste11, and Ste7. (2) Determine how the mating MAPK Fus3 prevents the filamentation MAPK, Kss1, from inappropriately activating genes regulated by the pheromone response pathway. (3) Elucidate why Ste12 activation by mating pheromone does not induce filamentation genes in wild-type cells but does so in fus3 knockout cells. (4) Use a reporter gene system to identify specificity factors and determine their mechanism of action. Because MAP kinase pathways are conserved between yeast and humans, the answers obtained in the yeast may reveal general principles common to all eukaryotes. Since cancer cells misinterpret environmental signals transduced by the MAP kinase cascade, understanding how signaling specificity is maintained may lead to a better understanding of this signaling derangement characteristic of tumor cells.