In Vitro Seamless Stack Enzymatic Assembly of DNA Molecules Based on a Strategy Involving Splicing of Restriction Sites

Cyclic Digestion and Ligation-Mediated PCR Used for Flanking Sequence Walking

Ligation-mediated PCR (LM-PCR) is a classical method for isolating flanking sequences; however, it has a common limitation of reduced success rate owing to the circularization or multimerization of target restriction fragments including the known sequence. To address this limitation, we developed a novel LM-PCR method, termed Cyclic Digestion and Ligation-Mediated PCR (CDL-PCR). The novelty of this approach involves the design of new adapters that cannot be digested after being ligated with the restriction fragment, and cyclic digestion and ligation may be manipulated to block the circularization or multimerization of the target restriction fragments. Moreover, to improve the generality and flexibility of CDL-PCR, an adapter precursor sequence was designed, which could be digested to prepare 12 different adapters at low cost. Using this method, the flanking sequences of T-DNA insertions were obtained from transgenic rice and Arabidopsis thaliana. The experimental results demonstrated that CDL-PCR is an efficient and flexible method for identifying the flanking sequences in transgenic rice and Arabidopsis thaliana.

A seamless and iterative DNA assembly method named PS-Brick and its assisted metabolic engineering for threonine and 1-propanol production

BACKGROUND

DNA assembly is an essential technique enabling metabolic engineering and synthetic biology. Combining novel DNA assembly technologies with rational metabolic engineering can facilitate the construction of microbial cell factories. Amino acids and derived biochemicals are important products in industrial biotechnology with wide application and huge markets. DNA assembly scenarios encountered in metabolic engineering for the construction of amino acid and related compound producers, such as design-build-test-learn cycles, construction of precise genetic circuits and repetitive DNA molecules, usually require for iterative, scarless and repetitive sequence assembly methods, respectively.

RESULTS

Restriction endonuclease (RE)-assisted strategies constitute one of the major categories of DNA assembly. Here, we developed a Type IIP and IIS RE-assisted method named PS-Brick that comprehensively takes advantage of the properties of PCR fragments and REs for iterative, seamless and repetitive sequence assembly. One round of PS-Brick reaction using purified plasmids and PCR fragments was accomplished within several hours, and transformation of the resultant reaction product from this PS-Brick assembly reaction exhibited high efficiency (104-105 CFUs/µg DNA) and high accuracy (~ 90%). An application of metabolic engineering to threonine production, including the release of feedback regulation, elimination of metabolic bottlenecks, intensification of threonine export and inactivation of threonine catabolism, was stepwise resolved in E. coli by rounds of "design-build-test-learn" cycles through the iterative PS-Brick paradigm, and 45.71 g/L threonine was obtained through fed-batch fermentation. In addition to the value of the iterative character of PS-Brick for sequential strain engineering, seamless cloning enabled precise in-frame fusion for codon saturation mutagenesis and bicistronic design, and the repetitive sequence cloning ability of PS-Brick enabled construction of tandem CRISPR sgRNA arrays for genome editing. Moreover, the heterologous pathway deriving 1-propanol pathway from threonine, composed of Lactococcus lactis kivD and Saccharomyces cerevisiae ADH2, was assembled by one cycle of PS-Brick, resulting in 1.35 g/L 1-propanol in fed-batch fermentation.

CONCLUSIONS

To the best of our knowledge, the PS-Brick framework is the first RE-assisted DNA assembly method using the strengths of both Type IIP and IIS REs. In this study, PS-Brick was demonstrated to be an efficient DNA assembly method for pathway construction and genome editing and was successfully applied in design-build-test-learn (DBTL) cycles of metabolic engineering for the production of threonine and threonine-derived 1-propanol. The PS-Brick presents a valuable addition to the current toolbox of synthetic biology and metabolic engineering.