Development of the CRISPR/Cas9 system for genome editing in Riemerella anatipestifer

Riemerella anatipestifer (R. anatipestifer) is an important pathogen responsible for high mortality rates and severe economic losses in the poultry industry. Research on R. anatipestifer is constrained by limited genetic manipulation tools, highlighting the need for an effective genome editing toolkit. The Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/Cas9 system from Streptococcus pyogenes has been widely used as a genome editing tool for various bacteria, but has not been used in R. anatipestifer. In this study, we developed a CRISPR/Cas9-based genome editing system, pCasRA-SacB, specifically for R. anatipestifer. This shuttle vector contains the replication origin from R. anatipestifer plasmid pRA0726 and p15A origin for replication in both R. anatipestifer and Escherichia coli (E. coli). It also includes chloramphenicol and cefoxitin resistance genes, which allow for selection in E. coli and R. anatipestifer. The vector features BsaI and SalI sites, enabling the cloning of single guide RNA (sgRNA) and homologous arms. In addition, it contains the high-expression promoter of B739_0921 gene and the promoter of rpsL gene, facilitating the expression of sgRNA and Cas9 protein. Furthermore, the vector features the sucrose-sensitive gene sacB and the oriT region, which enable plasmid curing and the transfer of plasmid via conjugation. Notably, the pCasRA-SacB system enables rapid, efficient, and scarless genome editing in R. anatipestifer, including gene deletion, insertion, and point mutation in the dprA gene, with editing efficiencies of 54.2 %, 100.0 %, and 50.0 %, respectively. In summary, the pCasRA-SacB system not only expands the genome editing toolbox of R. anatipestifer but also is helpful in fundamental research in R. anatipestifer.