FIG. 1.

Design of a haploid pheromone-responsive strain. (A) Experimental system. TAU3 cells (a1 mating type) express constitutively an ectopic copy of the pra2 gene, which encodes the a1 pheromone receptor (gray rectangles). Addition of minute amounts of a synthetic pheromone (black diamonds) induce the secretion of endogenous a1 pheromone (gray diamonds) that feeds back into the system in an autocrine manner and induces the mating program. (B) Induction of expression of the mfa1 gene, encoding a1 pheromone precursor. FB1c (control strain) and TAU3 (pheromone-responsive strain) cells were incubated at 28°C in the presence of pheromone at time zero. Aliquots were extracted at the indicated times, and total RNA was isolated and subjected to Northern analysis after loading 10 μg of total RNA per lane. The same filter was hybridized in succession with probes for mfa1 and 18S rRNA. (C) Mating structures induced by pheromone. FB1c (control strain) and TAU3 (pheromone-responsive strain) cells were incubated with (+a1) or without pheromone for 4 h. Conjugative tubes are visible only in pheromone-treated TAU3 cells. Bars, 5 μm.