Efficient Single-Nucleotide Microbial Genome Editing Achieved Using CRISPR/Cpf1 with Maximally 3'-End-Truncated crRNAs.

Mismatch tolerance, a cause of the off-target effect, impedes accurate genome editing with the CRISPR/Cas system. Herein, we observed that oligonucleotide-directed single-base substitutions could be rarely introduced in the microbial genome using CRISPR/Cpf1-mediated negative selection. Because crRNAs have the ability to recognize and discriminate among specific target DNA sequences, we systematically compared the effects of modified crRNAs with 3'-end nucleotide truncations and a single mismatch on the genomic cleavage activity of FnCpf1 in Escherichia coli . Five nucleotides could be maximally truncated at the crRNA 3'-end for the efficient cleavage of the DNA targets of galK and xylB in the cells. However, target cleavage in the genome was inefficient when a single mismatch was simultaneously introduced in the maximally 3'-end-truncated crRNA. Based on these results, we assumed that the maximally truncated crRNA-Cpf1 complex can distinguish between single-base-edited and unedited targets in vivo. Compared to other crRNAs with shorter truncations, maximally 3'-end-truncated crRNAs showed highly efficient single-base substitutions (>80%) in the DNA targets of galK and xylB . Furthermore, the editing efficiency for the 24 bases in both galK and xylB showed success rates of 79 and 50%, respectively. We successfully introduced single-nucleotide indels in galK and xylB with editing efficiencies of 79 and 62%, respectively. Collectively, the maximally truncated crRNA-Cpf1 complex could perform efficient base and nucleotide editing regardless of the target base location or mutation type; this system is a simple and efficient tool for microbial genome editing, including indel correction, at the single-nucleotide resolution.