Specific effects of triarimol on sterol biosynthesis in Ustilago maydis.

1. Triarimol (2 mug/ml) severely inhibited ergosterol synthesis in sporidia of Ustilago maydis. In control cells ergosterol accounted for 70-85% of the total sterols, In sporidia treated 9.5 h with triarimol the total sterol content was not appreciably reduced; however, ergosterol constituted less than 4% of the sterol fraction. In treated cells 95% of this fraction was composed of 24-methylene-dihydrolanosterol, obtusifoliol and 14alpha-methyl-delta-8,24(28)-ergostadienol. These three sterols are proposed as the first intermediates of ergosterol biosynthesis in U. maydis. 2. This study indicates that in U. maydis triarimol inhibits three reactions in ergosterol biosynthesis, all of which involve the D ring and the side chain of the sterol molecule: demethylation at C-14; introduction of the C-22(23) double bond; and reduction of the C-24(28) double bond. The nature of inhibition suggests triarimol may bind to a sterol-sterol carrier protein complex and prevent interaction of the complex with enzymes necessary to carry out reactions in the affected region. 3. Cells treated with triarimol for 9.5 h contained approximately 100 times the quantity of free fatty acids found in control cells. There were qualitative differences in free fatty acids and fatty acids of the di- and triglycerides in control and treated cells.