Transformed yeast cells that ferment pentose sugars
The present invention relates to host cells transformed with a nucleic acid sequence encoding a eukaryotic xylose isomerase obtainable from an anaerobic fungus. When expressed, the sequence encoding the xylose isomerase confers to the host cell the ability to convert xylose to xylulose which may be further metabolized by the host cell. Thus, the host cell is capable of growth on xylose as carbon source. The host cell preferably is a eukaryotic microorganism such as a yeast or a filamentous fungus. The invention further relates to processes for the production of fermentation products such as ethanol, in which a host cell of the invention uses xylose for growth and for the production of the fermentation product. The invention further relates to nucleic acid sequences encoding eukaryotic xylose isomerases and xylulose kinases as obtainable from anaerobic fungi.
1. A cultured yeast cell transformed with a nucleic acid expression construct which construct comprises:
(a) a nucleotide sequence that encodes xylose isomerase protein, the amino acid sequence of which is at least 90% identical with SEQ ID NO:1, and
(b) operative linked to the nucleotide sequence of (a), a promoter that drives active expression of the xylose isomerase coding sequence in the transformed cell,
wherein, said expression construct is expressible in said cell and expression thereof confers on the cell the ability to directly isomerize xylose to xylulose.
2. The cell of claim 1 that is a member of a genus selected from the group consisting of Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces , and Yarrowia.
3. The cell according to claim 2 that is a member of a species selected from the group consisting of S. cerevisiae, S. bulderi, S. barnetti, S. exiguus, S. uvarum, S. diastaticus, K. lactis, K. marxianus , and K. fragilis.
4. The cell according to claim 1 , wherein the xylose isomerase amino acid sequence encoded by said coding nucleotide sequence:
(A) comprises a catalytic triad including the following four residues at the indicated positions in SEQ ID NO:1: His 102 plus Asp 105, and Asp 340 and Lys 235; and,
(B) comprises at least one Mg-binding site that is residue Glu 233 of SEQ ID NO:1.
5. The cell according to claim 4 , wherein the xylose isomerase amino acid sequence that is encoded by said coding nucleotide sequence further comprises:
(C) a first xylose isomerase signature pattern defined by the presence of residues 185-194 of SEQ ID NO:1; and
(D) a second xylose isomerase signature pattern defined by the presence of residues 230-237 of SEQ ID NO:1.
6. The cell according to claim 1 , wherein the promoter is insensitive to catabolite repression in the cell.
7. The cell according to claim 1 that has been further genetically modified to confer on the cell one or more of the following properties:
(1) increased transport of xylose into the host cell;
(2) increased xylulose kinase activity;
(3) increased flux of the pentose phosphate pathway;
(4) decreased sensitivity to catabolite repression;
(5) increased tolerance to ethanol, osmolarity or organic acids; or
(6) decreased production of by-products,
in comparison to a similar cell that has not undergone said genetic modification.
8. The cell according to claim 7 , wherein the genetic modification that results in said properties (1)-(6) is
(A) overexpression of an endogenous gene,
(B) expression of a heterologous gene, or
(i) a pentose transporter;
(ii) a xylulose kinase;
(iii) an enzyme from the pentose phosphate pathway,
(iv) a glycolytic enzyme, or
(v) an ethanologenic enzyme.
9. The cell according to claim 7 , wherein the genetic modification that results in said properties (1)-(6) is one that causes inactivation of one of the following endogenous genes:
(a) a gene encoding a hexose kinase
(b) Saccharomyces MIG1 gene;
(c) Saccharomyces MIG2 gene; or
(d) a gene homologous to (a), (b) or (c) and which hybridizes thereto.
10. The cell according to claim 1 , that further expresses one or more enzymes that confers on the cell the ability to produce a non-ethanolic fermentation product.
11. The cell according to claim 10 , wherein said fermentation product is selected from the group consisting of lactic acid, acetic acid, succinic acid, amino acids, 1,3-propanediol, ethylene, and glycerol.
12. The cell according to claim 10 , wherein said fermentation product is a β-lactam antibiotic or a cephalosporin.
13. The cell according to claim 10 , in which alcohol dehydrogenase activity is genetically decreased to reduce ethanol production by said cell.