Mode of action of the azasteroid antibiotic 15-aza-24 methylene-d-homocholesta-8,14-dien-3 beta-ol in Ustilago maydis.

Ustilago maydis sporidia treated with 0.1 mug of azasterol (15-aza-24-methylene-d-homocholesta-8,14-dien-3beta-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-3beta-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.