Lactic acid production by Candida sonorensis

C. sonorensis readily ferments glucose to ethanol, utilizes several carbon sources other than glucose, and is tolerant to acidic conditions. C. sonorensis was engineered for lactate production by individually expressing different lactate dehydrogenase genes in pyruvate decarboxylase positive and negative strain backgrounds to generate strains with distinctive characteristics. Production of lactate and ethanol from glucose was strongly affected by the deletions of the two pyruvate decarboxylase genes PDC1 and PDC2, the properties of the LDH enzymes, and by LDH enzyme activity which varied with LDH gene copy number. C. sonorensis LDH strains were also shown to ferment xylose to lactic acid efficiently via the endogenous xylose pathway enzymes, e.g. via xylose reductase and xylitol dehydrogenase. An alternative xylose fermentation pathway was introduced into C. sonorensis by expressing the XYLA gene from Piromyces sp. encoding xylose isomerase, and xylose was also shown to be metabolized via this pathway. Co-expression of the XKS1 gene from Saccharomyces cerevisiae encoding xylulokinase with the XYLA gene enhanced fermentation relative to expression of XYLA alone. Deletion of the endogenous reductase and xylitol dehydrogenase genes from a strain expressing xylose isomerase had a positive effect on lactate production from xylose.