Pichia guilliermondii, Schwanniomyces occidentalis, Torulopsis candida and several riboflavin-dependent mutants of Torulopsis candida were grown in a medium with a low concentration of iron. In these conditions, the activity of GTP-cyclohydrolase which catalyzes the first step of flavinogenesis increases. The activity of the enzyme increases also when the cells of T. candida and P. guilliermondii with a high content of iron are incubated with alpha, alpha'-dipyridyl which induces overproduction of riboflavin; this action of alpha, alpha'-dipyridyl is eliminated by cycloheximide. Therefore, iron deficiency in the cells of these yeasts causes derepression of GTP-cyclohydrolase participating in riboflavin biosynthesis. The activity of the enzyme is inhibited by FAD but not by FMN and riboflavin.

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