Effect of differential expression of transaldolase gene on xylose fermentation and acetic acid tolerance of Saccharomyces cerevisiae strain

Abstract:

[Objective] Xylose fermentation is crucial in lignocellulosic ethanol production. Acetic acid generated during pretreatment process seriously inhibits xylose fermentation of yeast strain. The effect of differential expression of transaldolase gene (TAL1), one key gene in oxidative pentose phosphate pathway (PPP), on xylose utilization as well as acetic acid tolerance of genetically engineered xylose-fermenting strain NAPX37 was studied. [Methods] The promoter of TAL1 gene (PTAL1) of the strain NAPX37 was separately replaced with three promoters, PTDH3, PAHP1 and PUBI4, through homologous recombination. By subsequent sporulation, spore segregation and mating, three homozygotes in which PTAL1 were replaced with PTDH3, PAHP1 or PUBI4 were constructed. The fermentation capacity and acetic acid tolerance of the three homozygotes and the original strain NAPX37 were compared through batch fermentation using xylose or the mixture of glucose and xylose as carbon source. [Results] Three promoters, PTDH3, PAHP1 and PUBI4, increased the transcription level of TAL1 gene differentially, which not only improved xylose consumption rate and acetic acid tolerance significantly, but also improved glucose consumption rate under the condition of 60 mmol/L of acetic acid. When xylose was used as sole carbon source without acetic acid or when mixed sugar was used, the strain with PAHP1-controlled TAL1 gene showed better fermentation results than strains with PTDH3- or PUBI4-controlled TAL1 gene, indicating the expression level of PAHP1-controlled TAL1 gene was most appropriate. When xylose was used as sole carbon source under the condition of 30 mmol/L of acetic acid, the strain with PUBI4-controlled TAL1 gene showed best fermentation results among all strains, indicating the most suitable expression level of PUBI4-controlled TAL1 gene. [Conclusion] Three promoters, PTDH3, PAHP1 and PUBI4, overexpressed TAL1 gene, which improved xylose fermentation rate and acetic acid tolerance of strain NAPX37 differentially. However, the fermentation condition affected the level of improvement.