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Biotechnol Biofuels. 2014 May 22;7:75. doi: 10.1186/1754-6834-7-75. eCollection 2014.

Consolidated bioprocessing of transgenic switchgrass by an engineered and evolved Clostridium thermocellum strain.

Author information

1
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6341, USA ; BioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6037, USA.
2
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401, USA ; Current address: Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, No.189 Songling Rd, Qingdao City, Shandong Province 266101, People's Republic of China.
3
BioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6037, USA ; Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401, USA.
4
Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6341, USA ; BioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6037, USA ; White Cliff Biosystems, Rockwood, TN 37854, USA.

Abstract

BACKGROUND:

Switchgrass is an abundant and dedicated bioenergy feedstock, however its inherent recalcitrance is one of the economic hurdles for producing biofuels. The downregulation of the caffeic acid O-methyl transferase (COMT) gene in the lignin pathway of switchgrass reduced lignin content and S/G ratio, and the transgenic lines showed improved fermentation yield with Saccharomyces cerevisiae and wild-type Clostridium thermocellum (ATCC 27405) in comparison to the wild-type switchgrass.

RESULTS:

Here we examine the conversion and yield of the COMT transgenic and wild-type switchgrass lines with an engineered and evolved C. thermocellum (M1570) strain. The fermentation of the transgenic switchgrass by M1570 had superior conversion relative to the wild-type control switchgrass line with an increase in conversion of approximately 20% and ethanol being the primary product accounting for 90% of the total metabolites measured by HPLC analysis.

CONCLUSIONS:

The engineered and evolved C. thermocellum M1570 was found to respond to the apparent reduced recalcitrance of the COMT switchgrass with no substrate inhibition, producing more ethanol on the transgenic feedstock than the wild-type substrate. Since ethanol was the main fermentation metabolite produced by an engineered and evolved C. thermocellum strain, its ethanol yield on a transgenic switchgrass substrate (gram/gram (g/g) glucan liberated) is the highest produced thus far. This result indicates that the advantages of a modified feedstock can be combined with a modified consolidated bioprocessing microorganism as anticipated.

KEYWORDS:

Cellulosic ethanol; Clostridium thermocellum; Consolidated bioprocessing; Feedstock; Metabolic engineering; Switchgrass; Transgenic

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