With advances in recombinant DNA technology, a technique for obtaining gene products of interest has been developed. Such technique is realized by allowing foreign genes to be expressed in hosts such as microorganisms, molds, animals, plants, or insects and by allowing the resulting transformants to multiply. When a technique of yeast culturing is employed, for example, a large quantity of gene products of interest can be produced via fermentative production.
Effective production of lactic acid, which is a starting material of plant-derived plastics, has been awaited in recent years from the viewpoint of “carbon neutrality.”
Lactic acid is classified into L-lactic acid and D-lactic acid, which are optical isomers. A technique for fermentatively producing D-lactic acid has been known, whereby D-lactic acid is fermentatively produced using lactic acid bacteria capable of producing D-lactic acid in a medium containing brewers' yeast (JP Patent Publication (Kokai) No. 58-16688 A (1983)). Also, a technique that involves the use of Lactobacillus bulgaricus has been known (JP Patent Publication (Kokai) No. 58-36394 A (1983)). A technique for obtaining D-lactic acid with high optical purity has also been disclosed (JP Patent Publication (Kokai) Nos. 61-293387 A (1986), 4-271787 A (1992), and 2001-29063 (A)). Such microorganisms capable of producing lactic acid have not been suitable for industrial production of lactic acid due to the slow rates of production and the necessity for complicated medium compositions.
Another technique has also been disclosed, whereby a lactate dehydrogenase gene is incorporated into a yeast strain lacking the capacity for ethanol production or having a reduced capacity for ethanol production, and the resulting recombinant product is used to produce lactic acid (JP Patent Publication (Kokai) No. 2001-516584 (A)). This technique, however, merely discloses the production of L-lactic acid. Further, a technique for producing D-lactic acid has been disclosed, whereby a D-lactate dehydrogenase gene is introduced into yeast that has a high capacity for producing pyruvic acid (JP Patent Publication (Kokai) No. 2002-136293 (A)). This technique is, however, focused on highly concentrated pyruvic acid in a yeast strain having a high capacity for producing pyruvic acid. Accordingly, production by general yeast is not described therein.
The present invention is directed to providing a polynucleotide encoding a protein that can be utilized for producing D-lactic acid having lactate dehydrogenase activity, and it is also directed to providing such protein. In addition, the present invention is directed to providing an excellent system for producing D-lactic acid utilizing such polynucleotide and a technique for effectively producing D-lactic acid.