Cell lawns of S. cerevisiae strain DBY7442 [MATalpha; sst2] expressing the presumed PreA pheromone receptor were subjected to culture supernatants (sn) or Aspergillus minimal media with glucose (GMM) or xylose (XMM) as C-source to repress or induce, respectively, expression of the mating-type regulator MAT1-2-1 in strain AfS147 [pxylP::MAT1-2-1] and, therefore, secretion of a-factor-like pheromone. Growth arrest of the pheromone-sensitive tester strain accompanied by halo formation is evident when confronted with supernatant from the cultivation of AfS147 in inducing medium XMM (plate 8) but not in GMM (plate 9). As further controls, culture supernatants of AfS147 cultured in GMM or XMM were tested on a lawn of DBY7442 cells that do not express PreA (plates 2 and 3). Growth media themselves did not induce halo formation (plates 4-7), whereas conditioned medium containing S. cerevisiae a-pheromone served as positive control (plates 1 and 10).
