The rate and extent of a cell's response to an extracellular stimulus is influenced by regulators that act on the intracellular signalling machinery. Although not directly involved in propagating the intracellular signal, regulators control the activity of the proteins that transmit the signals. To understand this aspect of cell signalling, we studied the pheromone-response pathway in the fission yeast Schizosaccharomyces pombe, a relatively simple signalling system in a genetically tractable organism. Here, we describe the development of yeast strains containing ura4 and lacZ reporter genes under the control of the pheromone-regulated sxa2 promoter and the use of these strains to isolate mutants defective in their ability to regulate signalling. Several different types of mutant were identified. Some mutants were defective in proteins already known to regulate the pheromone-signalling pathway (Rgs1, Map1, Map2). Our approach also identified the MAP kinase phosphatase Pmp1 as a regulator of the pheromone-response pathway. Although previously shown to regulate other MAP kinase pathways in Sz. pombe, this is the first demonstration of a role for Pmp1 in pheromone signalling.
