Abstract
BackgroundDue to its cost-effectiveness and rich sugar composition, sugarcane molasses is considered to be a promising carbon source for biorefinery. However, the sugar mixture in sugarcane molasses is not consumed as efficiently as glucose in microbial fermentation due to complex interactions among their utilizing pathways, such as carbon catabolite repression (CCR). In this study, 2,3-butanediol-producing Enterobacter aerogenes was engineered to alleviate CCR and improve sugar utilization by modulating its carbon preference.ResultsThe gene encoding catabolite repressor/activator (Cra) was deleted in the genome of E. aerogenes to increase the fructose consumption rate. However, the deletion mutation repressed sucrose utilization, resulting in the accumulation of sucrose in the fermentation medium. Cra regulation on expression of the scrAB operon involved in sucrose catabolism was verified by reverse transcription and real-time PCR, and the efficiency of sucrose utilization was restored by disrupting the scrR gene and overexpressing the scrAB operon. In addition, overexpression of the ptsG gene involved in glucose utilization enhanced the glucose preference among mixed sugars, which relieved glucose accumulation in fed-batch fermentation. In fed-batch fermentation using sugarcane molasses, the maximum titer of 2,3-butanediol production by the mutant reached 140.0 g/L at 54 h, which was by far the highest titer of 2,3-butanediol with E. aerogenes achieved through genetic engineering.ConclusionsWe have developed genetically engineered E. aerogenes as a 2,3-butanediol producer that efficiently utilizes sugarcane molasses. The fermentation efficiency was dramatically improved by the alleviation of CCR and modulation of carbon preference. These results offer a metabolic engineering approach for achieving highly efficient utilization of mixed sugars for the biorefinery industry.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0290-3) contains supplementary material, which is available to authorized users.
Highlights
Due to its cost-effectiveness and rich sugar composition, sugarcane molasses is considered to be a promising carbon source for biorefinery
Sugarcane molasses was used as a cheap feedstock for 2,3-butanediol production with metabolically engineered E. aerogenes
For efficient utilization of sugarcane molasses, the removal of transcriptional repressors enabled the mutant strain to metabolize all sugars in sugarcane molasses simultaneously, which increased fermentation duration and 2,3-butanediol productivity
Summary
Due to its cost-effectiveness and rich sugar composition, sugarcane molasses is considered to be a promising carbon source for biorefinery. The sugar mixture in sugarcane molasses is not consumed as efficiently as glucose in microbial fermentation due to complex interactions among their utilizing pathways, such as carbon catabolite repression (CCR). 2,3-butanediol-producing Enterobacter aerogenes was engineered to alleviate CCR and improve sugar utilization by modulating its carbon preference. The presence of preferred carbon sources often prevents the utilization of secondary substrates by the regulatory mechanisms, including transcription activation or repression of certain genes concerned with the use of alternative carbon sources, which is called carbon catabolite repression (CCR) [11, 12]. Crp is known to regulate the genes involved in carbon metabolism, such as lactose, arabinose, mannose, glucosamine, and amino sugars operons, in response to the depletion of preferred carbon source [14, 15]. Because the catabolic characteristic of microbial fermentation with mixed sugars results in a delayed and complicated fermentation process, alleviation of CCR of a host microorganism is very helpful for improving its fermentation efficiency [17]
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