Parasitic reactions of the facultative biotrophic fusion parasites Absidia parricida, Chaetocladium brefeldii and Parasitella parasitica, order Mucorales, class Zygomycetes, are morphologically closely resembling the sexual reactions of that group. In the sexual process, partner recognition and sexual morphogenesis are mediated by signal molecules comprising several beta-carotene derived C18 compounds, the trisporoids, that are produced by both mating partners via a complex, cooperative synthesis pathway. As the parasitic interactions are confined to other members of the same group, and, in case of parasitism between Parasitella and one of its hosts, Absidia glauca, only occur between partners of complementary mating types, trisporoids are supposed to act as signal molecules in parasitism, too. We studied the regulation of 4-dihydromethyltrisporate dehydrogenase (TDH), a key enzyme in trisporoid biosynthesis, in Parasitella. The enzyme is active only in the minus mating type where it catalyzes the conversion of a plus-specific precursor. The TDH gene was found to display similar activities in sexual as well as in parasitic reactions of Parasitella. In both cases, the gene is transcribed in the plus and in the minus mating type, but the 32 kDa translation products only occur in the minus mating type. Histochemical analysis reveals high activity of the enzyme in parasitic interactions between Parasitella parasitica minus and a number of hosts belonging to different families within the Mucorales.
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