Selection and mutant properties of Pichia guilliermondii yeasts with derepressed GTP cyclohydrolase, the enzyme of the 1st step in flavinogenesis

A positive method is proposed for selecting Pichia guilliermondii mutants with derepressed GTP cyclohydrolase. Mutants with the incompletely blocked gene RIB2 were used as parent strains; these can grow in a medium without riboflavin (RF) only if the enzyme is derepressed as the result of iron deficiency in cells. Strains growing in a medium without RF at the optimal supply of cells with iron were selected as regulatory mutants. The mutants accumulated 6,7-dimethylpterin in high concentrations and a small amount of RF in the medium and in the cells. The activity of GTP cyclohydrolase rather than that of RF synthase increased in the mutants; the activity of RF kinase and FAD pyrophosphorylase was not elevated. Hybrids produced by crossing the regulatory mutants with wild type strains did not accumulate 6,7-dimethylpterin in the medium and the activity of the GTP cyclohydrolase did not increase; this is indicative of the negative regulation for the expression of the structural gene for GTP cyclohydrolase. The authors propose a model for the regulation of GTP cyclohydrolase and RF synthase at the gene level involving iron ions as a corepressor.