Metabolite stress and tolerance in the production of biofuels and chemicals: gene-expression-based systems analysis of butanol, butyrate, and acetate stresses in the anaerobe Clostridium acetobutylicum.

Metabolite accumulation has pleiotropic, toxic, or beneficial effects on cell physiology, but such effects are not well understood at the molecular level. Cells respond and adapt to metabolite stress by mechanisms largely unexplored, especially in the context of multiple and simultaneous stresses. Solventogenic and related clostridia have an inherent advantage for production of biofuels and chemicals directly from cellulosic material and other complex carbohydrates, but issues of product/metabolite tolerance and related culture productivities remain. Using DNA microarray-based gene expression analysis, the transcriptional-stress responses of Clostridium acetobutylicum to fermentation acids acetate and butyrate and the solvent product butanol were analyzed and compared in the context of cell physiology. Ontological analysis demonstrated that stress by all three metabolites resulted in upregulation of genes related to post-translational modifications and chaperone activity, and downregulation of the translation-machinery genes. Motility genes were downregulated by acetate-stress only. The general metabolite stress included upregulation of numerous stress genes (dnaK, groES, groEL, hsp90, hsp18, clpC, and htrA), the solventogenic operon aad-ctfA-ctfB, and other solventogenic genes. Acetate stress downregulated expression of the butyryl-CoA- and butyrate-formation genes, while butyrate stress downregulated expression of acetate-formation genes. Pyrimidine-biosynthesis genes were downregulated by most stresses, but purine-biosynthesis genes were upregulated by acetate and butyrate, possibly for thiamine and histidine biosynthesis. Methionine-biosynthesis genes were upregulated by acetate stress, indicating a possibly conserved stress response mechanism also observed in Escherichia coli. Nitrogen-fixation gene expression was upregulated by acetate stress. Butyrate stress upregulated many iron-metabolism genes, riboflavin-biosynthesis genes, and several genes related to cellular repair from oxidative stress, such as perR and superoxide dismutases. Butanol stress upregulated the glycerol metabolism genes glpA and glpF. Surprisingly, metabolite stress had no apparent effect on the expression of the sporulation-cascade genes. It is argued that the list of upregulated genes in response to the three metabolite stresses includes several genes whose overexpression would likely impart tolerance, thus making the information generated in this study, a valuable source for the development of tolerant recombinant strains.