Abstract
Background: Bioplastics, like polylactic acid (PLA), are
renewable alternatives for petroleum-based plastics.
Lactic acid, the monomer of PLA, has traditionally been
produced biotechnologically with bacteria. With genetic
engineering, yeast have the potential to replace bacteria
in biotechnological lactic acid production, with the
benefits of being acid tolerant and having simple
nutritional requirements. Lactate dehydrogenase genes
have been introduced to various yeast to demonstrate this
potential. Importantly, an industrial lactic acid
producing process utilising yeast has already been
implemented. Utilisation of D-xylose in addition to
D-glucose in production of biochemicals such as lactic
acid by microbial fermentation would be beneficial, as it
would allow lignocellulosic raw materials to be utilised
in the production processes.Results: The yeast Candida
sonorensis, which naturally metabolises D-xylose, was
genetically modified to produce L-lactic acid from
D-xylose by integrating the gene encoding L-lactic acid
dehydrogenase (ldhL) from Lactobacillus helveticus into
its genome. In microaerobic, CaCO3-buffered conditions a
C. sonorensis ldhL transformant having two copies of the
ldhL gene produced 31 g l-1 lactic acid from 50 g l-1
D-xylose free of ethanol.Anaerobic production of lactic
acid from D-xylose was assessed after introducing an
alternative pathway of D-xylose metabolism, i.e. by
adding a xylose isomerase encoded by XYLA from Piromyces
sp. alone or together with the xylulokinase encoding gene
XKS1 from Saccharomyces cerevisiae. Strains were further
modified by deletion of the endogenous xylose reductase
encoding gene, alone or together with the xylitol
dehydrogenase encoding gene. Strains of C. sonorensis
expressing xylose isomerase produced L-lactic acid from
D-xylose in anaerobic conditions. The highest anaerobic
L-lactic acid production (8.5 g l-1) was observed in
strains in which both the xylose reductase and xylitol
dehydrogenase encoding genes had been deleted and the
xylulokinase encoding gene from S. cerevisiae was
overexpressed.Conclusions: Integration of two copies of
the ldhL gene in C. sonorensis was sufficient to obtain
good L-lactic acid production from D-xylose. Under
anaerobic conditions, the ldhL strain with exogenous
xylose isomerase and xylulokinase genes expressed and the
endogenous xylose reductase and xylitol dehydrogenase
genes deleted had the highest L- lactic acid production.
Original language | English |
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Article number | 107 |
Journal | Microbial Cell Factories |
Volume | 13 |
DOIs | |
Publication status | Published - 2014 |
MoE publication type | A1 Journal article-refereed |
Keywords
- lactic acids
- biopolymers
- yeasts
- candida sonorensis
- xylose
- pyruvate decarboxylase
- xylose reductase
- xylitol dehydrogenase
- xylose isomerase