Organizational constraints on Ste12 cis-elements for a pheromone response in Saccharomyces cerevisiae.

Ste12 of Saccharomyces cerevisiae binds to pheromone-response cis-elements (PREs) to regulate several classes of genes. Genes induced by pheromones require multimerization of Ste12 for binding of at least two PREs on responsive promoters. We have systematically examined nucleotides of the consensus PRE for binding of wild-type Ste12 to DNA in vitro, as well as the organizational requirements of PREs to produce a pheromone response in vivo. Ste12 binds as a monomer to a single PRE in vitro, and two PREs upstream of a minimal core promoter cause induction that is proportional to their relative affinity for Ste12 in vitro. Although consensus PREs are arranged in a variety of configurations in the promoters of responsive genes, we find that there are severe constraints with respect to how they can be positioned in an artificial promoter to cause induction. Two closely-spaced PREs can induce transcription in a directly-repeated or tail-to-tail orientation, although PREs separated by at least 40 nucleotides are capable of inducing transcription when oriented in a head-to-head or tail-to-tail configuration. We characterize several examples of promoters that bear multiple consensus PREs or a single PRE in combination with a PRE-like sequence that match these requirements. A significant number of responsive genes appear to possess only a single PRE, or PREs in configurations that would not be expected to enable induction, and we suggest that, for many pheromone-responsive genes, Ste12 must activate transcription by binding to cryptic or sub-optimal sites on DNA, or may require interaction with additional uncharacterized DNA bound factors.