Addition of Multiple Introns to a Cas9 Gene Results in Dramatic Improvement in Efficiency for Generation of Gene Knockouts in Plants

The recent discovery of the mode of action of the CRISPR/Cas9 system has provided biologists with a useful tool for generating site-specific mutations in genes of interest. In plants, site-targeted mutations are usually obtained by stably transforming a Cas9 expression construct into the plant genome. The efficiency with which mutations are obtained in genes of interest can vary considerably depending on specific features of the constructs, including the source and nature of the promoters and terminators used for expression of the Cas9 gene and the guide RNA, and the sequence of the Cas9 nuclease itself. To optimize the efficiency with which mutations could be obtained in target genes in Arabidopsis thaliana with the Cas9 nuclease, we have investigated several features of its nucleotide and/or amino acid sequence, including the codon usage, the number of nuclear localization signals (NLS) and the presence or absence of introns. We found that the Cas9 gene codon usage had some effect on Cas9 activity and that two NLSs work better than one. However, the most important impact on the efficiency of the constructs was obtained by addition of 13 introns into the Cas9 coding sequence, which dramatically improved editing efficiencies of the constructs; none of the primary transformants obtained with a Cas9 lacking introns displayed a knockout mutant phenotype, whereas between 70% and 100% of primary transformants generated with intronized Cas9 displayed mutant phenotypes. The intronized Cas9 was also found to be effective in other plants such as Nicotiana benthamiana and Catharanthus roseus.