Bacterial expression, purification, and initial characterization of a full-length Cas13b protein from Porphyromonas gingivalis.

The CRISPR-Cas13b system is a recently identified Class 2, RNA-targeting CRISPR-Cas system. The system has been repurposed to achieve robust mRNA knockdown and precise RNA-editing in mammalian cells. While the CRISPR-Cas13b system has become a powerful tool for nucleic acids manipulation, the mechanisms of the system are still not fully understood. Cas13b endonucleases from different bacterial species show poor overall sequence homologies, suggesting that structural (and probably functional) diversities may exist. It is therefore important to study CRISPR-Cas13b cases from different bacterial species. Here we report the expression, purification, and initial characterization of a Cas13b endonuclease that is associated with the 8th putative CRISPR locus from Porphyromonas gingivalis genome (Pgi8Cas13b). The full-length Pgi8Cas13b protein (1119 residues) was successfully expressed in E. Coli cells, and purified by affinity and ion-exchange chromatography methods. The purified protein is biologically active, being able to bind its cognate crRNA with high specificity and affinity. Preparation of biologically active Pgi8Cas13b protein provides the basis for further in vitro biochemical and biophysical studies of the Pgi8Cas13b CRISPR system.