Mode of Action of the Azasteroid Antibiotic 15-Aza-24-Methylene- d -Homocholesta-8,14-Dien-3β-ol in Ustilago maydis

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Abstract
Ustilago maydis sporidia treated with 0.1 μg of azasterol (15-aza-24-methylene- d -homocholesta-8,14-dien-3β-ol) per ml appeared branched and vacuolated after 6 h of incubation. Sporidial multiplication, dry weight increase, and synthesis of protein, deoxyribonucleic acid, and ribonucleic acid were only slightly or moderately inhibited during the initial 3 h of incubation. An increase of free fatty acids was observed in lipid extracts of treated sporidia after incubation for 3 h or more. Ergosterol synthesis was completely inhibited within 1 h and there was a gradual decline of ergosterol content during 6 h which was accompanied by an accumulation of the sterol intermediate ergosta-8,14-dien-3β-ol. The results indicate that toxicity of the azasterol results from specific inhibition of the reduction of the sterol C-14(15) double bond. A triarimol-tolerant strain of Cladosporium cucumerinum was tolerant to the azasterol, but an imazalil-tolerant strain of Aspergillus nidulans was not.