Abstract
BackgroundThe yeast Saccharomyces cerevisiae is an important eukaryotic workhorse in traditional and modern biotechnology. At present, only a few S. cerevisiae strains have been extensively used as engineering hosts. Recently, an astonishing genotypic and phenotypic diversity of S. cerevisiae was disclosed in natural populations. We suppose that some natural strains can be recruited as superior host candidates in bioengineering. This study engineered a natural S. cerevisiae strain with advantages in inulin utilization to produce ethanol from inulin resources by consolidated bioprocess. Rational engineering strategies were employed, including secretive co-expression of heterologous exo- and endo-inulinases, repression of a protease, and switch between haploid and diploid strains.ResultsResults from co-expressing endo- and exo-inulinase genes showed that the extracellular inulinase activity increased 20 to 30-fold in engineered S. cerevisiae strains. Repression of the protease PEP4 influenced cell physiology in late stationary phase. Comparison between haploid and diploid engineered strains indicated that diploid strains were superior to haploid strains in ethanol production albeit not in production and secretion of inulinases. Ethanol fermentation from both inulin and Jerusalem artichoke tuber powder was dramatically improved in most engineered strains. Ethanol yield achieved in the ultimate diploid strain JZD-InuMKCP was close to the theoretical maximum. Productivity achieved in the strain JZD-InuMKCP reached to 2.44 and 3.13 g/L/h in fermentation from 200 g/L inulin and 250 g/L raw Jerusalem artichoke tuber powder, respectively. To our knowledge, these are the highest productivities reported up to now in ethanol fermentation from inulin resources.ConclusionsAlthough model S. cerevisiae strains are preferentially used as hosts in bioengineering, some natural strains do have specific excellent properties. This study successfully engineered a natural S. cerevisiae strain for efficient ethanol production from inulin resources by consolidated bioprocess, which indicated the feasibility of natural strains used as bioengineering hosts. This study also presented different properties in enzyme secretion and ethanol fermentation between haploid and diploid engineering strains. These findings provided guidelines for host selection in bioengineering.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0511-4) contains supplementary material, which is available to authorized users.
Highlights
The yeast Saccharomyces cerevisiae is an important eukaryotic workhorse in traditional and modern biotechnology
Secretive expression of endo‐ and exo‐inulinases in JZH The haploid S. cerevisiae strain JZH was derived from the ascospores of the diploid strain JZ1C by tetrad dissection
The endo-inulinase gene InuB of A. niger was integrated into the JZH genome at the loci of Ty1 under the control of the PTDH3 promoter and mating factor α (MFα) signal sequence, generating a strain JZH-tmInuB (Table 1)
Summary
The yeast Saccharomyces cerevisiae is an important eukaryotic workhorse in traditional and modern biotechnology. Only a few S. cerevisiae strains have been extensively used as engineering hosts. This study engineered a natural S. cerevisiae strain with advantages in inulin utilization to produce ethanol from inulin resources by consoli‐ dated bioprocess. The budding yeast Saccharomyces cerevisiae is an important cell factory for production of traditional foods, enzymes, and pharmaceuticals. Inulin is a naturally occurring storage polysaccharide present in numerous plants, such as Jerusalem artichoke and chicory. Jerusalem artichoke is treated as a sustainable feedstock for bioethanol production by consolidated bioprocess (CBP). CBP requires yeasts to complete the processes of inulinase production, inulin hydrolysis, and fermentation in one reactor [10, 17]. S. cerevisiae has an invertase SUC2 that possesses exo-inulinase activity, the yeast cannot efficiently convert inulin into ethanol because of the low activity of SUC2 toward inulin molecules with high degree of polymerization (DP) values [18, 19]
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