Abstract
Enterococcus mundtii QU25, a non-dairy lactic acid bacterium of the phylum Firmicutes, is capable of simultaneously fermenting cellobiose and xylose, and is described as a promising strain for the industrial production of optically pure l-lactic acid (≥ 99.9%) via homo-fermentation of lignocellulosic hydrolysates. Generally, Firmicutes bacteria show preferential consumption of sugar (usually glucose), termed carbon catabolite repression (CCR), while hampering the catabolism of other sugars. In our previous study, QU25 exhibited apparent CCR in a glucose-xylose mixture phenotypically, and transcriptional repression of the xylose operon encoding initial xylose metabolism genes, likely occurred in a CcpA-dependent manner. QU25 did not exhibit CCR phenotypically in a cellobiose-xylose mixture. The aim of the current study is to elucidate the transcriptional change associated with the simultaneous utilization of cellobiose and xylose. To this end, we performed RNA-seq analysis in the exponential growth phase of E. mundtii QU25 cells grown in glucose, cellobiose, and/or xylose as either sole or co-carbon sources. Our transcriptomic data showed that the xylose operon was weakly repressed in cells grown in a cellobiose-xylose mixture compared with that in cells grown in a glucose-xylose mixture. Furthermore, the gene expression of talC, the sole gene encoding transaldolase, is expected to be repressed by CcpA-mediated CCR. QU25 metabolized xylose without using transaldolase, which is necessary for homolactic fermentation from pentoses using the pentose-phosphate pathway. Hence, the metabolism of xylose in the presence of cellobiose by QU25 may have been due to 1) sufficient amounts of proteins encoded by the xylose operon genes for xylose metabolism despite of the slight repression of the operon, and 2) bypassing of the pentose-phosphate pathway without the TalC activity. Accordingly, we have determined the targets of genetic modification in QU25 to metabolize cellobiose, xylose and glucose simultaneously for application of the lactic fermentation from lignocellulosic hydrolysates.
Highlights
Pure lactic acid is a feedstock for the production of poly-lactic acid (PLA), a biobased and biodegradable plastic [1], and of lactate-based polyesters containing 3-hydroxyalkanoates [2]
We reported that the QU25 strain exhibits apparent carbon catabolite repression (CCR) in a glucose-xylose mixture, which is controlled at the transcriptional level with the repression occurring in initial xylose metabolism genes [11]
To examine the transcriptome of strain QU25 under various sugar conditions, we performed RNA-seq analysis on QU25 cultured in glucose, cellobiose, xylose, a glucose-xylose mixture, and a cellobiose-xylose mixture at the exponential growth phase (S1 Fig)
Summary
Pure lactic acid is a feedstock for the production of poly-lactic acid (PLA), a biobased and biodegradable plastic [1], and of lactate-based polyesters containing 3-hydroxyalkanoates [2]. The hydrolysates of the lignocellulosic biomass contain hexoses (predominant component, cellobiose, a β1,4-linked glucose dimer) and pentoses (primarily xylose and arabinose) [3]. Enterococcus mundtii QU25 is a non-dairy lactic acid bacterium, isolated from ovine feces that can ferment both cellobiose and xylose to produce optically pure L-lactic acid ( 99.9%) via homo-fermentation [3,4,5]. Comparison of the results of lactic acid production from xylose shows that QU25 is one of the most efficient lactic acid-producing bacteria, giving a productivity of 6.15 (gL-1h-1), yield of 1.01 (gg-1), and final concentration of 41.0 (gL-1) [3,6,7]. The industrial use of this strain has potential for facilitating the economical production of L-lactate from lignocellulosic biomass
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