Biosynthesis of flavins and its regulation in the yeast Pichia guilliermondii

The nature of riboflavin precursors was studied in the yeast Pichia guilliermondii. By means of mutants with blocked GMP-synthetase the purine precursors of riboflavin were shown to belong to guanylic compounds. Accumulation of 2,4,5-triamino-6-oxypyrimidine, 2,5-diamino-6-oxy-4-ribitylaminopyrimidine, 2,6-dioxy-5-amino-4-ribitylaminopyrimidine (DOARAP) and 6,7-dimethyl-8-ribityllumasine occurs in the riboflavin-deficient mutants divided into five biochemical groups. This fact evidences for identity of riboflavin precursors in the yeast P. guilliermondii and Saccharomyces cerevisiae. Synthesis of DOARAP by the washed off cells of the mutants with the blocked lumasine synthetase is strongly inhibited by riboflavin; cycloheximide in the absence of riboflavin has no effect on this process. Consequently, flavinogenesis in P. guilliermondii is regulated according to the type of negative feedback by means of retroinhibition mechanism. A change in the content of flavins in the cells has no effect on synthesis of riboflavin synthetase; at the same time iron deficiency in the cells evokes derepression of this enzyme. Incubation of the cells rich in iron with o-phenantroline or alpha, alpha'-dipyridyl also causes derepression of riboflav...Continue Reading