[The reversion of Pichia guilliermondii transformants to the wild-type phenotype]. / Zakonomernosti vosstanovleniia fenotipa dikogo tipa u transformantov drozhzhei Pichia guilliermondii.

Pichia guilliermondii strain with blocked GTP cyclohydrolase II was transformed using replicative plasmids and their fragments containing the structural gene RIB7 of this enzyme. Experiments showed that the presence of an ARS element and the promoter region of the gene in the genome of transformants reduces the probability of their reversion to the wild-type phenotype. Different types of recombination in the yeast P. guilliermondii are discussed.

Oversynthesis of riboflavin by yeast Pichia guilliermondii in response to oxidative stress.

The effect of oxidative stress on riboflavin (vitamin B2) biosynthesis and iron accumulation in flavinogenic yeast P. guilliermondii was investigated. Treatment of P. guilliermondii cells with superoxidgenerating agent methylviologen leads to elevated production of malondialdyhyd (MDA) which reflects the overall cellular oxidation state. Increased iron content in the cells and enhanced productivity of flavinogenesis under these conditions has been shown too. Significant increasing of MDA and riboflavin production by yeast cells under iron deficiency was observed. Riboflavin overproducing P. guilliermondii mutant strains rib80, rib81 and hit, possess high iron transport and synthesize increased quantity of MDA. The role of riboflavin overproduction and activation of iron assimilation in the P. guilliermondii antioxidant defence is discussed.

[Synergistic effect of rib80 and hit mutations on riboflavin biosynthesis and iron transport in the yeast Pichia guilliermondii]. / Sovmestnoe vliianie mutatsii rib80 and hit na biosintez riboflavina i transport zheleza u drozhzhei Pichia Guilliermondii.

Mutant strains of the yeast Pichia guilliermondii, carrying both rib80 and hit mutations in a haploid genome, were derived from previously obtained strains with defective rib80 or hit genes, exerting negative control of the riboflavin biosynthesis and iron transport in Pichia guilliermondii. The double mutant rib80hit strains exhibited an increased level of riboflavin biosynthesis and higher activities of GTP cyclohydrolase and riboflavin synthetase. Iron deficiency caused an additional increase in riboflavin overproduction. These results suggest the synergistic interaction of the rib80 and hit mutations. A combination of both mutations in a single genome did not affect iron assimilation by the cells: ferrireductase activity, the rate of 55Fe uptake, and the iron content in cells of the double mutants remained at the level characteristic of the parent strains.

[Ferrireductase from Pichia guilliermondii: properties and regulation of activity and synthesis]. / Ferireduktaza Pichia guilliermondii: vlastyvosti i rehuliarsiia aktyvnosti ta syntezu.

Properties of Pichia guilliermondii ferrireductase of the crude extracts and ammonium sulfate preparations were studied. NADH and Cu (II) ions are necessary for ferrireductase activity. Mg (II) also stimulates this reaction while oxygen acts as a slight inhibitor. Ferrireductase reduces Fe (III) in complex with citrate, iron-binding peptide from the culture medium of P. guilliermondii, rhodotorulic acid, coprogen, desferrioxamine B and EDTA. Mutants rib80, rib81 and hit of Pichia guilliermondii that have damaged system of riboflavin biosynthesis and iron transport regulation are characterized by high ferrireductase activity. Iron through negative feed-back mechanism regulates activity of ferrireductase synthesis in P. guilliermondii, Candida famata, Candida krusei, Candida boidinii, but not in Pichia pinus and Hansenula polymorpha.