Pentose sugar fermenting cell

1. A Saccharomyces cerevisiae cell comprising a heterologous nucleotide sequence encoding a polypeptide comprising at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 2 and having xylose isomerase activity, wherein said Saccharomyces cerevisiae cell can grow on xylose as sole carbon source under aerobic and anaerobic conditions.

2. The Saccharomyces cerevisiae cell of claim 1 , wherein said heterologous nucleotide sequence encoding said polypeptide having xylose isomerase activity is obtained from a cell of the genus Clostridium.

3. The Saccharomyces cerevisiae cell of claim 1 , wherein said Saccharomyces cerevisiae cell further comprises at least one genetic modification resulting in:

a. an increase in transport of xylose in the cell;

b. an increase in xylulose kinase activity;

c. an increase in flux through the pentose phosphate pathway;

d. a decrease in aldose reductase activity;

e. a decrease in sensitivity to catabolite repression;

f. an increase in tolerance to ethanol, osmolarity or organic acids; or

g. a reduced production of by-products, as compared with a Saccharomyces cerevisiae cell without said at least one genetic modification, wherein said at least one genetic modification is selected from the group consisting of overexpression of at least one gene encoding an enzyme of the non-oxidative part of the pentose phosphate pathway, overexpression of a gene encoding a xylulose kinase, expression of genes araA, araB and araD from Lactobacillus plantarum , inactivation of a gene encoding an endogenous unspecific aldose reductase, and a combination thereof.

4. The Saccharomyces cerevisiae cell to of claim 3 , wherein said at least one genetic modification is overexpression of at least one gene encoding an enzyme of the non-oxidative part of the pentose phosphate pathway.

5. The Saccharomyces cerevisiae cell to of claim 4 , wherein said gene is a gene encoding a ribulose-5-phosphate isomerase, a ribulose-5-phosphate epimerase, a transketolase, and/or a transaldolase.

6. The Saccharomyces cerevisiae cell of claim 3 , wherein said at least one genetic modification is overexpression of a gene encoding a xylulose kinase.

7. The Saccharomyces cerevisiae cell of claim 3 , wherein said at least one genetic modification is inactivation of a gene encoding an endogenous unspecific aldose reductase.

8. The Saccharomyces cerevisiae cell of claim 1 , wherein said Saccharomyces cerevisiae cell further comprises overexpression of TAL1, TKL1, RPE1 and RKI1 genes and the ability to use L-arabinose.

9. The Saccharomyces cerevisiae cell of claim 3 , comprising at least one constitutively expressed or constitutively overexpressed gene stably integrated into the genome of said Saccharomyces cerevisiae cell.

10. A process for producing a fermentation product comprising:

fermenting a medium containing xylose in the presence of the Saccharomyces cerevisiae cell of claim 1 under conditions whereby said fermentation product is produced.

11. The process of claim 10 , wherein said fermentation product is ethanol, butanol, lactic acid, 2-hydroxy-propionic acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, itaconic acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, a β-lactam antibiotic and/or a cephalosporin.

12. The process of claim 10 , wherein said process is anaerobic.

13. The Saccharomyces cerevisiae cell of claim 1 , wherein said heterologous nucleotide sequence encodes a polypeptide comprising at least 97% sequence identity to the amino acid sequence of SEQ ID NO: 2 and having xylose isomerase activity.