Mining microbial organisms to discover and characterize novel CRISPR-Cas systems

The need for new genome manipulation tools is leading the way for the continued discovery of novel clustered regularly interspaced short palindromic repeats— CRISPR associated sequences (CRISPR-Cas) systems. Researchers have been analyzing the genomes of prokaryotes and more recently metagenomic sequencing data to find novel and diverse CRISPR-Cas systems and their associated genome editing effectors. In this review, we provide an overview of in silico, in vitro, and in vivo analyses performed to characterize key elements of CRISPR-Cas systems, encompassing the CRISPR array, Cas proteins, guide ribonucleic acid (RNAs), and protospacer-adjacent motif (PAM) which defines targeting. We also highlight subsequent in vitro and in vivo assays employed to validate CRISPR function and Cas effector activity in the context of genome editing in various cellular contexts.