Summary: is a naturally occurring sterol auxotroph with an absolute nutritional requirement for one of a small group of structurally related phytosterols. We report here a quantitative study demonstrating that a low, otherwise sub-supportive, concentration (≈ 0.020-0.050 μg ml) of an essential phytosterol (stigmasterol) is adequate for growth of this ciliate, provided that a second, relatively non-specific sterol is available at a higher concentration (1.0 μg ml) to allow for membrane biosynthesis. This phenomenon, referred to as sterol synergism, has been observed in a broad taxonomic range of organisms, with the conclusion that small amounts of specific sterols are required to perform some previously unknown, vital metabolic or regulatory function. promises to be an excellent model organism for the elucidation of essential sterol function.


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