CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2018/041038 filed Nov. 5, 2018, which claims the benefit of priority based on Japanese Patent Application No. 2017-214102 filed Nov. 6, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a mutant gene associated with improvement in ethanol productivity in ethanol fermentation performed with a yeast strain or so on having xylose-metabolizing ability and a method for producing ethanol using the same.

BACKGROUND ART

A cellulosic biomass is an effective starting material for a useful alcohol, such as ethanol, or an organic acid. In order to increase the amount of ethanol produced with the use of a cellulosic biomass, yeast strains capable of utilizing xylose, which is pentose, as a substrate have been developed. For example, Patent Document 1 discloses a recombinant yeast strain resulting from integration of a xylose reductase (XR) gene and a xylitol dehydrogenase (XDH) gene derived fromPichia stipitisinto its chromosome. Also, a report has been made concerning a xylose-assimilating yeast strain into which a xylose isomerase (XI) gene derived from the intestinal protozoa ofReticulitermes speratushas been introduced (Patent Document 2).

An attempt for improving ethanol productivity of a xylose-assimilating yeast strain or a general yeast strain that produces ethanol from glucose via fermentation has been reported. For example, Patent Document 3 reports that the alcohol-producing capacity of sake yeast strain is improved upon introduction of a particular mutation into the PDR3 gene. Also, Patent Document 4 discloses a recombinantKlebsiella oxytoca, which is prepared by introducing alcohol dehydrogenase and pyruvate decarboxylase into a host and deleting a gene associated with production of butanediol or 2,3-butanediol therefrom. With the use of the recombinant disclosed in Patent Document 4, ethanol can be produced with high efficiency by converting a sugar-containing substrate in a medium mainly into ethanol and decreasing conversion thereof into butanediol or 2,3-butanediol.

Patent Document 5 discloses that a recombinant yeast strain comprising a xylose metabolism-associated expression cassette introduced thereinto is subjected to acclimatization to improve the xylose fermentation ability of the recombinant yeast strain. In addition, Patent Document 6 discloses a xylose isomerase gene derived from the intestinal protozoa ofReticulitermes speratusor the intestinal protozoa ofMastotermes darwiniensis. The xylose isomerase gene disclosed in Patent Document 6 effectively functions in yeast and it is capable of improving xylose-metabolizing ability of yeast.

PRIOR ART DOCUMENTS

Patent Documents

[Patent Document 1] JP 2009-195220 A[Patent Document 2] JP 2011-147445 A[Patent Document 3] JP 2002-238582 A[Patent Document 4] JP 2009-500035 A[Patent Document 5] JP 2009-195220 A[Patent Document 6] JP 2009-195220 A

SUMMARY OF THE INVENTION

Objects to be Attained by the Invention

Yeast strains having xylose-metabolizing ability were insufficient in terms of ethanol productivity from xylose in a medium. Under the circumstances described above, accordingly, the present invention is intended to improve ethanol fermentation ability of a yeast strain having xylose-metabolizing ability.

Means for Attaining the Objects

The present inventors have conducted concentrated studies in order to attain the above objects. As a result, they succeeded in obtaining a yeast strain with improved fermentation performance when subjecting a yeast strain with xylose-metabolizing ability to long-term continuous culture and identifying a plurality of mutations associated with excellent ethanol fermentation ability of the yeast strain via thorough analysis of the obtained strain. This has led to the completion of the present invention.

The present invention includes the following.

(1) A mutant gene encoding the mutant CDC123 protein comprising a consensus sequence comprising a substitution of an amino acid residue in the 30th position from the N terminus with another amino acid residue in SEQ ID NO: 1.

(2) The mutant gene according to (1), wherein the amino acid residue in the 30th position is any of leucine, valine, and isoleucine and the another amino acid residue is cysteine.

(3) The mutant gene according to (1), wherein the mutant CDC123 protein comprises:

(a) the amino acid sequence as shown in SEQ ID NO: 3; or

(b) an amino acid sequence having 70% or higher identity to the amino acid sequence of SEQ ID NO: 3 in which an amino acid residue corresponding to the 324th position from the N terminus of the amino acid sequence of SEQ ID NO: 3 is cysteine.

(4) A mutant gene encoding the SUI3 protein comprising a consensus sequence comprising a substitution of an amino acid residue in the 52nd position from the N terminus with another amino acid residue in SEQ ID NO: 4.

(5) The mutant gene according to (4), wherein the amino acid residue in the 52nd position is serine or asparagine and the another amino acid residue is threonine.

(6) The mutant gene according to (4), wherein the mutant SUI3 protein comprises:

(a) the amino acid sequence as shown in SEQ ID NO: 6; or

(b) an amino acid sequence having 70% or higher identity to the amino acid sequence of SEQ ID NO: 6 in which an amino acid residue corresponding to the 112th position from the N terminus of the amino acid sequence of SEQ ID NO: 6 is threonine.

(7) A mutant gene encoding the mutant FPK1 protein comprising a consensus sequence comprising a substitution of an amino acid residue in the 31st position from the N terminus with another amino acid residue in SEQ ID NO: 7.

(8) The mutant gene according to (7), wherein the amino acid residue in the 31st position is glycine and the another amino acid residue is glutamic acid.

(9) The mutant gene according to (7), wherein the mutant FPK1 protein comprises:

(a) the amino acid sequence as shown in SEQ ID NO: 9; or

(b) an amino acid sequence having 70% or higher identity to the amino acid sequence of SEQ ID NO: 9 in which an amino acid residue corresponding to the 704th position from the N terminus of the amino acid sequence of SEQ ID NO: 9 is glutamic acid.

(10) A mutant yeast strain having xylose-metabolizing ability, which comprises the mutant gene according to any of (1) to (9).

(11) A method for producing ethanol comprising a step of culturing the mutant yeast strain according to (10) in a xylose-containing medium and performing ethanol fermentation.

(12) The method for producing ethanol according to (11), wherein the medium contains cellulose and the ethanol fermentation proceeds simultaneously at least with the cellulose saccharification.

This description includes part or all of the content as disclosed in the description and/or drawings of Japanese Patent Application No. 2017-214102, which is a priority document of the present application.

Effects of the Invention

The mutant gene according to the present invention is capable of imparting a yeast strain having xylose-metabolizing ability with excellent ethanol fermentation ability. Specifically, the mutant yeast strain according to the present invention can exhibit excellent ethanol fermentation ability. Accordingly, the mutant gene according to the present invention, a mutant yeast strain comprising such mutant gene, and a method for producing ethanol using the same can provide excellent ethanol productivity.

