To determine whether Fus3p occupies a specific set of genes upon activation, we performed genome-wide ChIP-Chip with a yeast strain containing a TAP-tagged Fus3p before and after pheromone exposure (Fig 2).
Fus3p, Kss1p and Ste5p occupy 9, 8 and 12 pheromone-induced genes within 5 min after exposure to mating pheromone (P < 0.001).
Occupancy was observed throughout the transcribed regions of these target genes (Fig. 2B).
The set of genes that are occupied by Fus3p, Kss1p and Fus3p and transcriptionally activated during mating have previously been shown to be dependant on the pheromone MAPK pathway for their expression (Roberts et al. 2000).
We did not find evidence that the Ste7p MAPKK and Ste11p MAPKKK occupy chromatin in independent genome-wide ChIP-Chip experiments, which is consistent with the cytoplasmic localization of Ste7p and Ste11p during activation by pheromone (Huh et al. 2003).
These results suggest that Ste5p may function not only as an adaptor for protein-protein interactions at the plasma membrane but also in the nucleus, where it may mediate association of Fus3p and Kss1p with chromatin.