The ascomycetous yeast Yarrowia lipolytica (formerly Candida, Endomycopsis, or Saccharomyces lipolytica) is one of the more intensively studied 'non-conventional' yeast species. This yeast is quite different from the well-studied yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe with respect to its phylogenetic evolution, physiology, genetics, and molecular biology. However, Y. lipolytica is not only of interest for fundamental research, but also for biotechnological applications. It secretes several metabolites in large amounts (i.e. organic acids, extracellular proteins) and the tools are available for overproduction and secretion of foreign proteins. This review presents a comprehensive overview on the available data on physiology, cell biology, molecular biology and genetics of Y. lipolytica.

Physiology and genetics of the dimorphic fungus Yarrowia lipolytica

This review provides a description of the biochemistry and enzymology of the α-aminoadipate pathway for lysine biosynthesis in fungi. The α-aminoadipate pathway is unique to fungi and is thus a potential target for the rational design of antifungal drugs. The present state of knowledge of the mechanisms of the seven enzymes in the pathway is presented, as well as detailed information with respect to structures and mechanisms of homocitrate synthase, saccharopine reductase, and saccharopine dehydrogenase.

The α-aminoadipate pathway for lysine biosynthesis in fungi

(1985). α-Aminoadipate Pathway for the Biosynthesis of Lysine in Lower Eukaryotes. CRC Critical Reviews in Microbiology: Vol. 12, No. 2, pp. 131-151.

alpha-Aminoadipate pathway for the biosynthesis of lysine in lower eukaryotes.

A Yarrowia lipolytica yeast gene bank has been constructed in E. coli in an integrative plasmid vector containing the homologous LEU2 gene, and used to transform a leu2 lys5 yeast strain. The LYS5 gene encoding saccharopine dehydrogenase (SDH) has been rescued in E. coli from the chromosome of prototrophic transformants, in which the hybrid plasmid had integrated at the leu2 locus. Evidence that the rescued clone contains the LYS5 gene comes from complementation tests, genetic crosses, and SDH assay. Further characterization of the gene has been achieved by deletion mapping and subcloning, and by demonstrating the presence of transcripts hybridizing to this sequence.

Cloning of the LYS5 gene encoding saccharopine dehydrogenase from the yeast Yarrowia lipolytica by target integration

Expression of the structural genes for lysine biosynthesis responds to an induction mechanism mediated by the transcriptional activator Lys14p in the presence of alpha-aminoadipate semialdehyde (alphaAASA), an intermediate of the pathway acting as a coinducer. This activation is reduced by the presence of lysine in the growth medium, leading to apparent repression. In this report we demonstrate that Saccharomyces cerevisiae possesses two genes, LYS20 and LYS21, encoding two homocitrate synthase isoenzymes which are located in the nucleus. Each isoform is inhibited by lysine with a different sensitivity. Lysine-overproducing mutants were isolated as resistant to aminoethylcysteine, a toxic lysine analog. Mutations, LYS20fbr and LYS21fbr, are allelic to LYS20 and LYS21, and lead to desensitization of homocitrate synthase activity towards lysine and to a loss of apparent repression by this amino acid. There is a fair correlation between the I0.5 of homocitrate synthase for lysine, the intracellular lysine pool and the levels of Lys enzymes, confirming the importance of the activity control of the first step of the pathway for the expression of LYS genes. The data are consistent with the conclusion that inhibition by lysine of Lys14p activation results from the control of alphaAASA production through the feedback inhibition of homocitrate synthase activity.

In Saccharomyces cerevisae, feedback inhibition of homocitrate synthase isoenzymes by lysine modulates the activation of LYS gene expression by Lys14p.

C.M. Gaillardin

Papers

A kinetic study of homocitrate synthetase activity in the yeast Saccharomycopsis lipolytica.