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Appl. Environ. Microbiol., 12 1995, 4184-4190, Vol 61, No. 12
M Walfridsson, J Hallborn, M Penttila, S Keranen and B Hahn-Hagerdal
Saccharomyces cerevisiae was metabolically engineered for xylose
utilization. The Pichia stipitis CBS 6054 genes XYL1 and XYL2 encoding
xylose reductase and xylitol dehydrogenase were cloned into S. cerevisiae.
The gene products catalyze the two initial steps in xylose utilization
which S. cerevisiae lacks. In order to increase the flux through the
pentose phosphate pathway, the S. cerevisiae TKL1 and TAL1 genes encoding
transketolase and transaldolase were overexpressed. A XYL1- and
XYL2-containing S. cerevisiae strain overexpressing TAL1 (S104-TAL) showed
considerably enhanced growth on xylose compared with a strain containing
only XYL1 and XYL2. Overexpression of only TKL1 did not influence growth.
The results indicate that the transaldolase level in S. cerevisiae is
insufficient for the efficient utilization of pentose phosphate pathway
metabolites. Mixtures of xylose and glucose were simultaneously consumed
with the recombinant strain S104-TAL. The rate of xylose consumption was
higher in the presence of glucose. Xylose was used for growth and xylitol
formation, but not for ethanol production. Decreased oxygenation resulted
in impaired growth and increased xylitol formation. Fermentation with
strain S103-TAL, having a xylose reductase/xylitol dehydrogenase ratio of
0.5:30 compared with 4.2:5.8 for S104-TAL, did not prevent xylitol
formation.
Copyright © 1995, American Society for Microbiology
Xylose-metabolizing Saccharomyces cerevisiae strains overexpressing the TKL1 and TAL1 genes encoding the pentose phosphate pathway enzymes transketolase and transaldolase
Department of Applied Microbiology, Lund University, Sweden.
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