Expression systems in eukaryotic microorganisms are useful for the production of various proteins (including human proteins), with respect to a greater probability of proper folding compared with Escherichia coli expression systems. The budding yeast Saccharomyces cerevisiae is a good host for eukaryotic expression systems, since various methods of genetic engineering are available, and well-organized genome information is available. Only a few genetic elements (e.g. promoters and signal peptides) are utilized in existing yeast expression systems; however, a large number of genetic elements in the S. cerevisiae genome remain unused. Therefore, we want to develop efficient expression systems for the production of useful materials by yeasts using the various genetic elements contained in the yeast genome.

1. Development of efficient protein expression systems in yeast using its genomic information: In order to provide a useful expression system for human and mammalian proteins, we are constructing a unique and powerful low temperature S. cerevisiae expression system at. This system is optimally designed based on the comprehensive analysis of gene expression in yeast, as well as bioinformatics using yeast genome information. When enhanced green fluorescent protein (GFP) was expressed in this system at a low temperature, its volume reached approximately 50% of the soluble protein in yeast cells, surpassing the existing normal temperature yeast expression systems. Furthermore, we confirm that our expression system can produce many human proteins in a soluble form, which is difficult using existing expression systems. We are now improving our expression system to produce various other proteins.
2. Development of expression systems for efficient xylose metabolism in bioethanol-producing yeast: S. cerevisiae has naturally high ethanol production ability, and is widely used for ethanol production from glucose. However, wild-type S. cerevisiae cannot ferment xylose, which is the dominant pentose sugar in plant biomass hydrolysates. One strategy to engineer yeasts capable of efficiently producing ethanol from xylose is by introducing the genes involved in xylose metabolism. However, optimization of the expression levels of xylose metabolic enzyme genes for efficient xylose metabolism in yeast has not been investigated in detail. Therefore, we want to improve the xylose to ethanol conversion efficiency in yeast by controlling expression levels of xylose metabolic enzyme genes using various promoters in the yeast genome. We are also attempting to develop expression systems for conferring thermo- and low pH-tolerance to a bioethanol-producing yeast by comparative genome analyses of thermo- and low pH-tolerant yeast strains using the next-generation sequencer.