Genetically Modified Yeast of the Species Issatchenkia Orientalis and Closely Related Species, and Fermentation Processes Using Same

Abstract

Cells of the species Issatchenkia orientalis and closely related yeast species are transformed with a vector to introduce an exogenous lactate dehydrogenase gene. The cells produce lactic acid efficiently and are resistant at low pH, high lactate titer conditions.

1 . A genetically modified yeast cell of a species within the  I. orientalis/P. fermentans  clade having at least one exogenous lactate dehydrogenase (LDH) gene. 
     
     
         2 . The yeast cell of  claim 1  which is of the species  Issatchenkia orientalis, Pichia galeiformis, Pichia  sp. YB-4149 (NRRL designation),  Candida ethanolica, P. deserticola, P. membranifaciens  or  P. fermentans.    
     
     
         3 . The yeast cell of  claim 1  wherein the exogenous LDH gene is integrated into the genome of the yeast cell. 
     
     
         4 . (canceled) 
     
     
         5 . The yeast cell of  claim 1 , wherein the exogenous LDH gene is a functional LDH gene that encodes a protein at least 80% identical to that encoded by a functional L-LDH gene of any of the species  Lactobacillus helveticus, L. casei, Bacillus megaterium, Pediococcus acidilactici, Bos taurus  or  Rhizopus oryzae.    
     
     
         6 - 8 . (canceled) 
     
     
         9 . The yeast cell  claim 1  which has a deletion or disruption of a native pyruvate decarboxylase (PDC) gene. 
     
     
         10 . The yeast cell of  claim 9  which is unable to produce ethanol. 
     
     
         11 . The yeast cell of  claim 1  wherein the exogenous LDH gene is under the transcriptional control of a promoter that is native to a yeast cell. 
     
     
         12 . The yeast cell of  claim 11  wherein the promoter is native to the host cell. 
     
     
         13 . The yeast cell of  claim 1  wherein the LDH gene is integrated at a locus of a native PDC gene. 
     
     
         14 . The yeast cell of  claim 1  wherein the host cell is  I. orientalis  or  P. membranifaciens.    
     
     
         15 . The yeast cell of  claim 9  wherein the host cell is  I. orientalis  or  P. membranifaciens.    
     
     
         16 . The yeast cell of  claim 10  wherein the host cell is  I. orientalis  or  P. membranifaciens.    
     
     
         17 . The yeast cell of  claim 1  further having at least one further genetic modification selected from (1) an insertion of a functional exogenous xylose isomerase gene, (2) a deletion or disruption of a native gene that produces an enzyme that catalyzes the conversion of xylose to xylitol, (3) a deletion or disruption of a functional xylitol dehydrogenase gene and/or (4) a modification that causes the cell to overexpress a functional xylulokinase. 
     
     
         18 - 19 . (canceled) 
     
     
         20 . A fermentation process in which a genetically modified yeast cell of  claim 1  is cultured under fermentation conditions in a fermentation broth that includes a fermentable sugar to produce lactic acid or a salt thereof. 
     
     
         21 . (canceled) 
     
     
         22 . The process of  claim 20  wherein the pH of the fermentation broth during at least a portion of the period of fermentation is in the range of from about 1.5 to about 4.5. 
     
     
         23 - 28 . (canceled) 
     
     
         29 . The process of  claim 20 , further comprising recovering lactate from the fermentation broth. 
     
     
         30 . The process of  claim 29 , further comprising producing lactide from the recovered lactate. 
     
     
         31 . The process of  claim 30 , further comprising polymerizing the lactide to from a poly(lactide) polymer. 
     
     
         32 - 45 . (canceled)