Andriy Sibirny: Peroxisomes are important for xylose alcoholic fermentation in the thermotolerant methylotrophic yeast Hansenula polymorpha
Large Seminar Room, Muthgasse 18, 1190 Vienna
Institute of Cell Biology, NAS of Ukraine
Diethard Mattanovich - acib
Hansenula polymorpha belongs to the best studied non-conventional yeasts, its genome has been sequenced and publicly available (http://genomeportal.jgipsf.org/Hanpo2/Hanpo2.info.html) and methods of molecular genetics and cell biology are well developed. It is a popular host system for expression of heterologous proteins and some metabolites as well as favorite model organism for studying peroxisome biogenesis and autophagic degradation, methanol catabolism and stress response. Besides, H. polymorpha is apparently the most thermotolerant yeast known with maximal growth temperature at 50 oC. Several years ago, we have found that H. polymorpha is capable of high-temperature xylose alcoholic fermentation though ethanol production is quite low. At the same time is effectively ferments glucose and cellobiose (Ryabova et al., 2003, Ishchuk et al., 2009). More recently, it was shown that H. polymorpha can effectively produce ethanol from glycerol, the byproduct of biodiesel production (Suvannarangsee et al., 2010). Using methods of classical selection and metabolic engineering, the strains of H. polymorpha have been constructed which accumulate elevated amounts of ethanol from xylose, up to 10-12 g/L. For this, overexpression of the modified gene XYL1m (encoding xylose reductase) and the native genes XYL2 (xylitol dehydrogenase), XYL3 (xylulokinase) and PDC1 (pyruvate decarboxylase) and additionally, the isolation of 3-bromopyruvate-resistant mutants from metabolically engineered strains have been conducted (Dmytruk et al., 2008; Ishchuk et al., 2008). However, to be economically feasible, available level of ethanol synthesis from xylose has to be increased for several times, up to 30-35 g/L. It was unexpectedly found by us that the pex mutants of the yeast Hansenula polymorpha which do not contain peroxisomes poorly grow and do not ferment xylose. The mutants of this yeast deleted in genes DAS1 and TAL2 coding for peroxisomal dihydroxyacetone synthase and transketolase, respectively, show normal growth in xylose medium though fermentation of this sugar is severely damaged. Inversely, overexpression of DAS1 and TAL2 resulted in activation of xylose alcoholic fermentation whereas overexpression of cytosolic genes of pentose phosphate pathway TKT1, TAL1, RPI1 and RPE1have only slight effect on this process. Defect in peroxisomes autophagic degradation (pexophagy) due to insertion or deletion of the gene ATG13 activated xylose alcoholic fermentation. All mentioned genetic changes have no effect on glucose alcoholic fermentation. The role of peroxisomes in xylose alcoholic fermentation is discussed.