Sugar uptake by the solventogenic clostridia

Abstract

The acetone-butanol-ethanol fermentation of solventogenic clostridia was operated as a successful, worldwide industrial process during the first half of the twentieth century, but went into decline for economic reasons. The recent resurgence in interest in the fermentation has been due principally to the recognised potential of butanol as a biofuel, and development of reliable molecular tools has encouraged realistic prospects of bacterial strains being engineered to optimise fermentation performance. In order to minimise costs, emphasis is being placed on waste feedstock streams containing a range of fermentable carbohydrates. It is therefore important to develop a detailed understanding of the mechanisms of carbohydrate uptake so that effective engineering strategies can be identified. This review surveys present knowledge of sugar uptake and its control in solventogenic clostridia. The major mechanism of sugar uptake is the PEP-dependent phosphotransferase system (PTS), which both transports and phosphorylates its sugar substrates and plays a central role in metabolic regulation. Clostridial genome sequences have indicated the presence of numerous phosphotransferase systems for uptake of hexose sugars, hexose derivatives and disaccharides. On the other hand, uptake of sugars such as pentoses occurs via non-PTS mechanisms. Progress in characterization of clostridial sugar transporters and manipulation of control mechanisms to optimise sugar fermentation is described.

Keywords: ABE fermentation; Catabolite repression; Phosphotransferase system; Sugar uptake.

Fig. 1

Bacterial sugar transport mechanisms. A H⁺-symport, in which the uptake of sugar occurs concurrently with a proton, driven by a transmembrane proton gradient, B Na⁺-symport, in which the uptake of sugar occurs concurrently with a sodium ion, driven by a transmembrane sodium gradient, C ABC (ATP-binding cassette) system supporting sugar uptake via ATP hydrolysis, D PEP-dependent phosphotransferase system coupling sugar uptake to phosphorylation. S = sugar

Fig. 2

Phylogenetic tree of characterised bacterial hexose and pentose transporters belonging to the major facilitator superfamily. Clostridial protein sequences CacXylT (Cac1345), Cac1339, Cac1530, Cac3422 and Cac3451 of C. acetobutylicum and CbeXylT (Cbe0109) and Cbe4545 of C. beijerinckii were obtained from the C. acetobutylicum (http://www.ncbi.nlm.nih.gov/nuccore/15893298) and C. beijerinckii (http://www.ncbi.nlm.nih.gov/nuccore/150014892?report=genbank) genome websites. Other sequences included in the analysis are as follows: BloGlcP, Bifidobacterium longum GlcP WP_011068757.1 (Parche et al. 2006); BsuAraE, Bacillus subtilis AraE WP_003243899.1 (Krispin and Allmansberger 1998); BsuGlcP, B. subtilis GlcP WP_003245772.1 (Paulsen et al. 1998); CglAraE, Corynebacterium glutamicum AraE BAH60837.1 (Sasaki et al. 2009); EcoAraE, Escherichia coli AraE WP_000256438.1 (Daruwalla et al. ; Hasona et al. 2004); EcoFucP, E. coli FucP WP_000528603.1 (Bradley et al. 1987); EcoGalP, E. coli GalP WP_001112301.1 (Henderson et al. ; Hernández-Montalvo et al. 2001); EcoXylE, E. coli XylE WP_001097274.1 (Lam et al. ; Sun et al. 2012); LbrXylT, Lactobacillus brevis XylT O52733.1 (Chaillou et al. 1998); MsmGlcP, Mycobacterium smegmatis GlcP WP_011729622.1 (Pimentel-Schmitt et al. 2009); PanFucP, Pantoea asanatis FucP WP_013024528.1 (Andreeva et al., 2013); PanGalP, P. asanatis GalP WP_014594508.1 (Andreeva et al. 2013); PanXylE, P. asanatis XylE WP_014593545.1 (Andreeva et al. 2013); RjoGluP, Rhodococcus jostii GluP WP_009475028.1 (Araki et al. 2011); ScoGlcP, Streptomyces coelicolor GlcP WP_003971990.1 (van Wezel et al. 2005); SynGlcP, Synechocystis sp. PCC6803 GlcP WP_010873345.1 (Zhang et al. 1989); ZmoGlf, Zymomonas mobilis Glf WP_011240287.1 (Weisser et al. 1995). Sugars which have been identified as substrates are indicated as follows (D-isomers unless otherwise indicated): ara, arabinose; fru, fructose; gal, galactose; glc, glucose; man, mannose; xyl, xylose; 2DG, 2-deoxyglucose; 6DG, 6-deoxyglucose; αMG, methyl-α-glucoside. Multiple alignment of protein sequences was performed using Clustal Omega of the European Bioinformatics Institute (http://www.ebi.ac.uk/Tools/msa/clustalo/) and phylogenetic trees were drawn using TreeView (Page 1996). Scale bar = 0.1 amino acid substitution per site

Fig. 3

Control of sugar metabolism by glucose in firmicutes. A CCR in which CcpA is induced to bind to a target DNA sequence by a seryl-phosphorylated form of HPr (P ~ Ser-HPr) and fructose 1,6-bisphosphate (FBP). B Control of pts operon expression via PTS-dependent phosphorylation of an antiterminator or transcriptional activator. See text for details