Saccharomyces cerevisiae Engineered for Xylose Metabolism Exhibits a Respiratory Response

Abstract

Native strains of Saccharomyces cerevisiae do not assimilate xylose. S. cerevisiae engineered for D-xylose utilization through the heterologous expression of genes for aldose reductase (XYL1), xylitol dehydrogenase (XYL2), and D-xylulokinase (XYL3 or XKS1) produce only limited amounts of ethanol in xylose medium. In recombinant S. cerevisiae expressing XYL1, XYL2, and XYL3, mRNA transcript levels for glycolytic, fermentative, and pentose phosphate enzymes did not change significantly on glucose or xylose under aeration or oxygen limitation. However, expression of genes encoding the tricarboxylic acid cycle, respiration enzymes (HXK1, ADH2, COX13, NDI1, and NDE1), and regulatory proteins (HAP4 and MTH1) increased significantly when cells were cultivated on xylose, and the genes for respiration were even more elevated under oxygen limitation. These results suggest that recombinant S. cerevisiae does not recognize xylose as a fermentable carbon source and that respiratory proteins are induced in response to cytosolic redox imbalance; however, lower sugar uptake and growth rates on xylose might also induce transcripts for respiration. A petite respiration-deficient mutant (°) of the engineered strain produced more ethanol and accumulated less xylitol from xylose. It formed characteristic colonies on glucose, but it did not grow on xylose. These results are consistent with the higher respiratory activity of recombinant S. cerevisiae when growing on xylose and with its inability to grow on xylose under anaerobic conditions.

Progress in Metabolic Engineering of Saccharomyces cerevisiae

“…Significant improvements were also achieved by applying evolutionary engineering to the strains previously equipped with one of the two heterologous pathways (185,382). Potential limitations for efficient ethanol production from xylose by engineered S. cerevisiae are, for example, the redox imbalance of the "yeast pathway" (138,302), a low xylose uptake rate (185,308), an insufficient ATP production rate (324), and the fact that xylose does not, unlike glucose, induce fermentative catabolism in yeast (146,302).…”

Limitations in Xylose-FermentingSaccharomyces cerevisiae, Made Evident through Comprehensive Metabolite Profiling and Thermodynamic Analysis

“…Therefore, this implies a catabolic function for the PP pathway during pentose utilization. Evidence from different studies in which the yeast transcriptome and proteome were analyzed under different substrate (glucose, xylose) conditions suggests that S. cerevisiae is not well adapted for fueling lower glycolysis via the PP pathway (24,28,(51)(52)(53)(54). It also appeared from the "omics" data that xylose was poorly "sensed" as a substrate for alcoholic fermentation.…”

Paenibacillus sp. Strain JDR-2 and XynA 1 : a Novel System for Methylglucuronoxylan Utilization

“…Supplementing the energy infrastructure with ethanol may help to shift economic dependence from petroleum-based energy. Microbial biocatalysts, both yeast and bacteria, have been developed for the conversion of glucose derived from cellulose and pentoses derived from hemicellulose to ethanol (22,35,37,38), and similar approaches with bacteria have been successfully applied to the formation of value-added products such as optically pure lactic acid (23,66,67). Current research efforts are directed at improving the pretreatment processes to maximize the release of fermentable pentoses as well as glucose and to further develop bacterial biocatalysts for specific fermentations.…”