Project description:Increasing demand for the production of renewable fuels has recently generated a particular interest in microbial production of butanol. Anaerobic bacteria, such as Clostridium spp., can naturally convert carbohydrates into a variety of primary products, including alcohols like butanol. The genetics of microorganisms like Clostridium acetobutylicum have been well studied and their solvent-producing metabolic pathways characterized. In contrast, less is known about the genetics of Clostridium spp. capable of converting syngas or its individual components into solvents. In this study, the type of strain of a new solventogenic Clostridium species, C. carboxidivorans, was genetically characterized by genome sequencing. C. carboxidivorans strain P7(T) possessed a complete Wood-Ljungdahl pathway gene cluster, involving CO and CO(2) fixation and conversion to acetyl-CoA. Moreover, with the exception of an acetone production pathway, all the genetic determinants of canonical ABE metabolic pathways for acetate, butyrate, ethanol and butanol production were present in the P7(T) chromosome. The functionality of these pathways was also confirmed by growth of P7(T) on CO and production of CO(2) as well as volatile fatty acids (acetate and butyrate) and solvents (ethanol and butanol). P7(T) was also found to harbour a 19 Kbp plasmid, which did not include essential or butanol production related genes. This study has generated in depth knowledge of the P7(T) genome, which will be helpful in developing metabolic engineering strategies to improve C. carboxidivorans's natural capacity to produce potential biofuels from syngas.
Project description:Clostridium carboxidivorans P7 (DSM 15243) is a bacterium that converts syngas (a mixture of CO, H2, and CO2) into hexanol. An optimized and scaled-up industrial process could therefore provide a renewable source of fuels and chemicals while consuming industry waste gases. However, the genetic engineering of this bacterium is hindered by its multiple restriction-modification (RM) systems: the genome of C. carboxidivorans encodes at least ten restriction enzymes and eight methyltransferases (MTases). To gain insight into the complex RM systems of C. carboxidivorans, we analyzed genomic methylation patterns using single-molecule real-time (SMRT) sequencing and bisulfite sequencing. We identified six methylated sequence motifs. To match the methylation sites to the predicted MTases of C. carboxidivorans, we expressed them individually in Escherichia coli for functional characterization. Recognition motifs were identified for all three Type I MTases (CAYNNNNNCTGC/GCAGNNNNNRTG, CCANNNNNNNNTCG/CGANNNNNNNNTGG and GCANNNNNNNTNNCG/CGNNANNNNNNNTGC), two Type II MTases (GATAAT and CRAAAAR), and a single Type III MTase (GAAAT). However, no methylated recognition motif was found for one of the three Type II enzymes. One recognition motif that was methylated in C. carboxidivorans but not in E. coli (AGAAGC) was matched to the remaining Type III MTase through a process of elimination. Understanding these enzymes and the corresponding recognition sites will facilitate the development of genetic tools for C. carboxidivorans that can accelerate the industrial exploitation of this strain.
Project description:BackgroundClostridium carboxidivorans P7 is capable of producing ethanol and butanol from inexpensive and non-food feedstock, such as syngas. Achieving improved ethanol and butanol production in the strain for industrial application depends on the energetics and biomass, especially ATP availability.ResultsThis study found that exogenous addition of citrulline promoted accumulation of ATP, increased specific growth rate, and reduced the doubling time of C. carboxidivorans P7. In heterotrophic fermentation experiments, the addition of citrulline increased intracellular ATP by 3.39-fold, significantly enhancing the production of total alcohol (ethanol + butanol) by 20%. Moreover, in the syngas fermentation experiments, the addition of citrulline improved the level of intracellular ATP and the biomass by 80.5% and 31.6%, respectively, resulting in an 18.6% and 60.3% increase in ethanol and the alcohol/acid production ratio, respectively.ConclusionsThis is the first report that citrulline could promote the growth of C. carboxidivorans P7 and increase the level of intracellular ATP, which is of great significance for the use of C. carboxidivorans P7 to synthesize biofuels.
Project description:Higher alcohols such as butanol (C4 alcohol) and hexanol (C6 alcohol) are superior biofuels compared to ethanol. Clostridium carboxidivorans P7 is a typical acetogen capable of producing C4 and C6 alcohols natively. In this study, the composition of trace metals in culture medium was adjusted, and the effects of these adjustments on artificial syngas fermentation by C. carboxidivorans P7 were investigated. Nickel and ferrous ions were essential for growth and metabolite synthesis during syngas fermentation by P7. However, a decreased dose of molybdate improved alcohol fermentation performance by stimulating carbon fixation and solventogenesis. In response to the modified trace metal composition, cells grew to a maximum OD600 nm of 1.6 and accumulated ethanol and butanol to maximum concentrations of 2.0 and 1.0 g/L, respectively, in serum bottles. These yields were ten-fold higher than the yields generated using the original composition of trace metals. Furthermore, 0.5 g/L of hexanol was detected at the end of fermentation. The results from gene expression experiments examining genes related to carbon fixation and organic acid and solvent synthesis pathways revealed a dramatic up-regulation of the Wood-Ljungdahl pathway (WLP) gene cluster, the bcs gene cluster, and a putative CoA transferase and butanol dehydrogenase, thereby indicating that both de novo synthesis and acid re-assimilation contributed to the significantly elevated accumulation of higher alcohols. The bdh35 gene was speculated to be the key target for butanol synthesis during solventogenesis.
Project description:The production of hexanol from syngas by acetogens has gained attention as a replacement for petroleum-derived hexanol, which is widely used in the chemical synthesis and plastic industries. However, acetogenic bacteria generally produce C2 compounds (e.g., acetate and ethanol) as the main products. In this study, the gas fermentation conditions favorable for hexanol production were investigated at different temperatures (30-37°C) and CO gas contents (30-70%) in batch gas fermentation. Hexanol production increased from 0.02 to 0.09 g/L when the cultivation temperature was lowered from 37 to 30°C. As the CO content increased from 30 to 70%, the CO consumption rate and hexanol production (yield, titer, and ratio of C6 compound to total products) increased with the CO content. When 70% CO gas was repeatedly provided by flushing the headspace of the bottles at 30°C, the total alcohol production increased to 4.32 g/L at the expense of acids. Notably, hexanol production (1.90 g/L) was higher than that of ethanol (1.20 g/L) and butanol (1.20 g/L); this is the highest level of hexanol produced in gas fermentation to date and the first report of hexanol as the main product. Hexanol production was further enhanced to 2.34 g/L when 2 g/L ethanol was supplemented at the beginning of 70% CO gas refeeding fermentation. Particularly, hexanol productivity was significantly enhanced to 0.18 g/L/day while the supplemented ethanol was consumed, indicating that the conversion of ethanol to acetyl-CoA and reducing equivalents positively affected hexanol production. These optimized culture conditions (gas fermentation at 30°C and refeeding with 70% CO gas) and ethanol supplementation provide an effective and sustainable approach for bio-hexanol production.