One step forward, two steps back: Transcriptional advancements and fermentation phenomena in Actinobacillus succinogenes 130Z

Comparison of hydrothermolysis and mild-alkaline pretreatment methods on enhancing succinic acid production from hydrolyzed corn fiber

In the present work, mild alkaline pretreatments using either sodium hydroxide (0.05 g/g corn fiber) or calcium hydroxide (lime) (0.05 g/g corn fiber) were optimized and compared with hydrothermolysis pretreatment to enhance bioproduction of succinic acid from hydrolyzed corn fiber. The concentration, yield, and productivity of succinic acid from sodium hydroxide corn fiber hydrolysate (SH-CFH) were 14.0 g/L, 0.63 g/g sugars, and 0.47 g/L*h, respectively, while the concentration, yield, and productivity of succinic acid from hydrothermolysis-pretreated corn fiber hydrolysate (H-CFH) were 30.2 g/L, 0.71 g/g sugars, and 1.01 g/L*h, respectively. Very little succinic acid production (<1 g/L) was observed from lime pretreated corn fiber hydrolysate (L-CFH). When SH-CFH was supplemented only with yeast extract, succinic acid concentration was enhanced to 15.2 g/L with a yield of 0.64 g/g sugars, and productivity of 0.51 g/L*h. In this study, succinic acid concentration and productivity from H-CFH both increased by 8.6% and an succinic acid yield from sugars increased 1.2 times when compared to succinic acid production from H-CFH in a previous study in our lab.

Effects of down-regulation of ackA expression by CRISPR-dCpf1 on succinic acid production in Actinobacillus succinogenes

Succinic acid (SA), a key intermediate in the cellular tricarboxylic acid cycle (TCA), is a 4-carbon dicarboxylic acid of great industrial value. Actinobacillus succinogenes can ferment various carbon sources and accumulate relatively high concentrations of SA, but few reliable genetic engineering tools exist for A. succinogenes and this has hindered strain improvement to increase SA production for industrial application. Two different repressors, endonuclease-deactivated Cas9 (dCas9) from Streptococcus pyogenes and Cpf1 (dCpf1) from Francisella tularensis, were applied to construct a CRISPRi system in A. succinogenes. Codon-optimized Cas9 and native Cpf1 were successfully expressed in A. succinogenes, and the corresponding sgRNA and crRNA expression elements, promoted by the fumarate reductase promoter, frd, were introduced into the CRISPRi plasmid. The highest repression of the ackA gene (encoding acetate kinase) and thereby acetic acid production (~ eightfold) was achieved by the dCpf1-based CRISPRi system, in which the mutation site, E1006A acted at the start of the coding region of ackA, the gene which regulates acetic acid biosynthesis. Compared with the ackA gene knockout mutant, cell growth was moderately improved and SA production increased by 6.3%. Further, the SA titer and productivity in a 3 L fermenter reached 57.06 g/L and 1.87 g/L/h, and there was less acetic acid production. A dCpf1-based CRISPRi-mediated gene repression system was successfully established for the first time, providing a simple and effective tool for studying functional genomics in A. succinogenes and optimizing SA production.