Abstract
BackgroundThe recombinant Saccharomyces cerevisiae strains that acquired the ability to utilize xylose through metabolic and evolutionary engineering exhibit good performance when xylose is the sole carbon source in the medium (designated the X stage in the present work). However, the xylose consumption rate of strains is generally low after glucose depletion during glucose–xylose co-fermentation, despite the presence of xylose in the medium (designated the GX stage in the present work). Glucose fermentation appears to reduce the capacity of these strains to “recognize” xylose during the GX stage, a phenomenon termed the post-glucose effect on xylose metabolism.ResultsTwo independent xylose-fermenting S. cerevisiae strains derived from a haploid laboratory strain and a diploid industrial strain were used in the present study. Their common characteristics were investigated to reveal the mechanism underlying the post-glucose effect and to develop methods to alleviate this effect. Both strains showed lower growth and specific xylose consumption rates during the GX stage than during the X stage. Glycolysis, the pentose phosphate pathway, and translation-related gene expression were reduced; meanwhile, genes in the tricarboxylic acid cycle and glyoxylic acid cycle demonstrated higher expression during the GX stage than during the X stage. The effects of 11 transcription factors (TFs) whose expression levels significantly differed between the GX and X stages in both strains were investigated. Knockout of THI2 promoted ribosome synthesis, and the growth rate, specific xylose utilization rate, and specific ethanol production rate of the strain increased by 17.4, 26.8, and 32.4%, respectively, in the GX stage. Overexpression of the ribosome-related genes RPL9A, RPL7B, and RPL7A also enhanced xylose utilization in a corresponding manner. Furthermore, the overexpression of NRM1, which is related to the cell cycle, increased the growth rate by 8.7%, the xylose utilization rate by 30.0%, and the ethanol production rate by 76.6%.ConclusionsThe TFs Thi2p and Nrm1p exerted unexpected effects on the post-glucose effect, enhancing ribosome synthesis and altering the cell cycle, respectively. The results of this study will aid in maintaining highly efficient xylose metabolism during glucose–xylose co-fermentation, which is utilized for lignocellulosic bioethanol production.
Highlights
The recombinant Saccharomyces cerevisiae strains that acquired the ability to utilize xylose through metabolic and evolutionary engineering exhibit good performance when xylose is the sole carbon source in the medium
The metabolic activities of strains in the GX stage were much lower than those in the X stage To describe the mechanism underlying the post-glucose effect on xylose metabolism, two engineered xyloseutilizing S. cerevisiae strains with different genetic backgrounds, BSGX001 and XH7, were selected as test strains
For BSGX001, the specific growth rate, xylose consumption rate, and ethanol production rate during the GX stage were 78.5, 30.4, and 48.1% lower than those calculated for the X stage, respectively
Summary
The recombinant Saccharomyces cerevisiae strains that acquired the ability to utilize xylose through metabolic and evolutionary engineering exhibit good performance when xylose is the sole carbon source in the medium (designated the X stage in the present work). The xylose consumption rate of strains is generally low after glucose depletion during glucose–xylose co-fermentation, despite the presence of xylose in the medium (designated the GX stage in the present work). Saccharomyces cerevisiae is a robust and safe microorganism with a strong metabolism, and it is frequently used as a cell factory in the fermentation industry, for ethanol production. S. cerevisiae is considered the most promising microorganism that produces ethanol from lignocellulosic materials [5, 6]. The resultant strains demonstrated a basic capacity to convert xylose into ethanol via sequential xylulose-5-phosphate, PPP, and glycolysis steps [7]. The xylose conversion rate of these engineered strains significantly increased after a long cultivation time in medium with xylose as the sole carbon source [5, 7, 14,15,16]
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