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genes involved in G1 arrest of cells, cell fusion, and nuclear fusion. Mutations in genes in any of these pathways
cause defects in mating. As Fig. A.15 shows, G / G transmits the signal initiated by pheromone binding through a series of three protein kinases, known collectively as a MAP kinase cascade.
In the pheromone response pathway, the trio of kinases ﬁrst
phosphorylate each other and then promote phosphorylation of the Dig1 and Dig2 proteins which inhibit the STE12
gene product. Phosphorylation of Dig1/Dig2 causes them
to release Ste12, the transcription factor. Since transcription occurs in the nucleus, the activation of Ste12 shows
that the signal transduction pathway has transduced the
pheromone-induced signal from the plasma membrane to
the nucleus. Ste12 activates the transcription of several
genes whose products contribute to cell fusion (for example, FUS1) and nuclear fusion (for example, KAR5). All
these processes are necessary for mating. Components of
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- Spring '08