Fig. 1. Isolation of secreted molecules from the Hydra head organizer in a yeast signal peptide secretion screen. (A) Polyps of the temperature-sensitive strain Hydra magnipapillata sf-1 were exposed to heat shock for 3 days, causing the elimination of interstitial cells. Heads, as well as regenerating tips, were isolated from the heat-shocked polyps at various times after head removal for mRNA and cDNA synthesis. (B) Cloning strategy. Size-fractionated cDNA was cloned adjacent to a signal peptide-deficient yeast invertase gene for expression in the yeast strain YTK12. Clones expressing a fusion protein with an intact Hydra signal peptide were selected by growth on raffinose plates.