Evidence from cell-free systems for differences in the sterol biosynthetic pathway of Rhizoctonia solani and Phytophthora cinnamomi

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Abstract
Cell-free preparations of both R. solani, a sterol-synthesizing fungus, and P. cinnamomi, a non-sterol-synthesizing fungus, incubated in the presence of [2-14C]mevalonate and iodoacetamide, converted the mevalonate into labeled mevalonate 5-phosphate, mevalonate 5-pyrophosphate and isopentenyl pyrophosphate. In the absence of iodoacetamide, but under anaerobic conditions, the same preparations converted the mevalonate into labeled geraniol, farnesol and squalene, the first 2 compounds presumably as their pyrophosphates. When cell-free preparations of both organisms were incubated aerobically in the presence of [1-14C]isopentenyl pyrophosphate, only labeled geraniol, farnesol and squalene were recovered from the P. cinnamomi reaction mixture, whereas labeled geraniol, farnesol, squalene, squalene epoxide, lanosterol and ergosterol were present in the R. solani reaction mixture. When these same preparations were incubated in the presence of 14C-labeled squalene, labeled squalene epoxide, lanosterol and ergosterol were recovered from the R. solani reaction mixture. In contrast, the P. cinnamomi preparation was unable to convert the squalene into products further along the sterol pathway; instead, a portion of the labeled squalene was converted into water-soluble products, indicating the possible existence of a squalene-degradation process in this organism. It appears that the block in the sterol biosynthetic pathway of P. cinnamomi occurs at the level of squalene epoxidation.