[Biosynthesis and transport of sterols in the yeast Saccharomyces cerevisiae].

The yeast Saccharomyces cerevisiae was a powerful tool in the identification of the structural genes involved in sterol biosynthesis in eucaryotes. Among 20 genes, 16 were isolated by genetic techniques using either complementation of mutants or overexpression strategy using specific inhibitors. In spite of this good knowledge concerning the genes of the pathway, little is known about the regulation of the isoprenoid/steroid biosynthetic pathway. However, the existence of two genes encoding HMG-CoA reductase in yeast genome suggests strongly that this enzyme could play a fundamental function in regulation, such as in plants and mammals. The regulation mechanisms could also involve sterol trafficking and storage. Indeed, one enzyme in the pathway, the sterol-C24-methyl transferase is localized in lipid particles that correspond to the storage form of steryl esters. Yeast cells are impermeable towards exogenous sterols in aerobiosis and become permeable in anaerobiosis when ergosterol synthesis is precluded by the absence of molecular oxygen. This phenomenon called aerobic sterol exclusion is dependent on the hem status of the cell. One gene, named SUT1 was identified that directs aerobic sterol uptake in yeast SUT1 gene and his partner SUT2 present strong features common to yeast transcription factors and could regulate the expression of genes involved in sterol uptake or intracellular trafficking.