Breeding of a xylose-fermenting hybrid strain by mating genetically engineered haploid strains derived from industrial Saccharomyces cerevisiae

Engineering the xylose metabolism in Schizochytrium sp. to improve the utilization of lignocellulose

BACKGROUND

Schizochytrium sp. is a heterotrophic, oil-producing microorganism that can efficiently produce lipids. However, the industrial production of bulk chemicals using Schizochytrium sp. is still not economically viable due to high-cost culture medium. Replacing glucose with cheap and renewable lignocellulose is a highly promising approach to reduce production costs, but Schizochytrium sp. cannot efficiently metabolize xylose, a major pentose in lignocellulosic biomass.

RESULTS

In order to improve the utilization of lignocellulose by Schizochytrium sp., we cloned and functionally characterized the genes encoding enzymes involved in the xylose metabolism. The results showed that the endogenous xylose reductase and xylulose kinase genes possess corresponding functional activities. Additionally, attempts were made to construct a strain of Schizochytrium sp. that can effectively use xylose by using genetic engineering techniques to introduce exogenous xylitol dehydrogenase/xylose isomerase; however, the introduction of heterologous xylitol dehydrogenase did not produce a xylose-utilizing engineered strain, whereas the introduction of xylose isomerase did. The results showed that the engineered strain 308-XI with an exogenous xylose isomerase could consume 8.2 g/L xylose over 60 h of cultivation. Xylose consumption was further elevated to 11.1 g/L when heterologous xylose isomerase and xylulose kinase were overexpressed simultaneously. Furthermore, cultivation of 308-XI-XK(S) using lignocellulosic hydrolysates, which contained glucose and xylose, yielded a 22.4 g/L of dry cell weight and 5.3 g/L of total lipid titer, respectively, representing 42.7 and 30.4% increases compared to the wild type.

CONCLUSION

This study shows that engineering of Schizochytrium sp. to efficiently utilize xylose is conducive to improve its utilization of lignocellulose, which can reduce the costs of industrial lipid production.

Effects of pyruvate decarboxylase (pdc1, pdc5) gene knockout on the production of metabolites in two haploid Saccharomyces cerevisiae strains

Saccharomyces cerevisiae has good reproductive ability in both haploid and diploid forms, a pyruvate decarboxylase plays an important role in S. cerevisiae cell metabolism. In this study, pdc1 and pdc5 double knockout strains of S. cerevisiae H14-02 (MATa type) and S. cerevisiae H5-02 (MATα type) were obtained by the Cre/loxP technique. The effects of the deletion of pdc1 and pdc5 on the metabolites of the two haploid S. cerevisiae strains were consistent. In S. cerevisiae H14-02, the ethanol conversion decreased by 30.19%, the conversion of glycerol increased by 40.005%, the concentration of acetic acid decreased by 43.54%, the concentration of acetoin increased by 12.79 times, and the activity of pyruvate decarboxylase decreased by 40.91% compared to those in the original H14 strain. The original S. cerevisiae haploid strain H14 produced a small amount of acetoin but produced very little 2,3-butanediol. However, S. cerevisiae H14-02 produced 1.420 ± 0.063 g/L 2,3-BD. This study not only provides strain selection for obtaining haploid strains with a high yield of 2,3-BD but also lays a foundation for haploid S. cerevisiae to be used as a new tool for genetic research and breeding programs.

Whole-Genome Sequence of an Isogenic Haploid Strain, Saccharomyces cerevisiae IR-2idA30(MATa), Established from the Industrial Diploid Strain IR-2

We present the draft genome sequence of an isogenic haploid strain, IR-2idA30(MATa), established from Saccharomyces cerevisiae IR-2. Assembly of long reads and previously obtained contigs from the genome of diploid IR-2 resulted in 50 contigs, and the variations and sequencing errors were corrected by short reads.

We present the draft genome sequence of an isogenic haploid strain, IR-2idA30(MATa), established from Saccharomyces cerevisiae IR-2. Assembly of long reads and previously obtained contigs from the genome of diploid IR-2 resulted in 50 contigs, and the variations and sequencing errors were corrected by short reads.

Genetic improvement of xylose metabolism by enhancing the expression of pentose phosphate pathway genes in Saccharomyces cerevisiae IR-2 for high-temperature ethanol production

The pentose phosphate pathway (PPP) plays an important role in the efficiency of xylose fermentation during cellulosic ethanol production. In simultaneous saccharification and co-fermentation (SSCF), the optimal temperature for cellulase hydrolysis of lignocellulose is much higher than that of fermentation. Successful use of SSCF requires optimization of the expression of PPP genes at elevated temperatures. This study examined the combinatorial expression of PPP genes at high temperature. The results revealed that over-expression of TAL1 and TKL1 in Saccharomyces cerevisiae (S. cerevisiae) at 30 °C and over-expression of all PPP genes at 36 °C resulted in the highest ethanol productivities. Furthermore, combinatorial over-expression of PPP genes derived from S. cerevisiae and a thermostable yeast Kluyveromyces marxianus allowed the strain to ferment xylose with ethanol productivity of 0.51 g/L/h, even at 38 °C. These results clearly demonstrate that xylose metabolism can be improved by the utilization of appropriate combinations of thermostable PPP genes in high-temperature production of ethanol.