Class 2 type VI-B CRISPR-associated RNA-guided ribonuclease Cas13b
CRISPR-Cas (clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins) adaptive immune systems defend microbes against foreign nucleic acids via RNA-guided endonucleases. These systems are divided into two classes; class 1 systems utilize multiple Cas proteins and CRISPR RNA (crRNA) to form an effector complex while class 2 systems employ a large, single effector with crRNA to mediate interference. Class 2 type VI CRISPR-Cas13 systems use a single enzyme to target RNA using a programmable crRNA guide and are divided into four subtypes based on the identity of the Cas13 protein (Cas13a-d). The Cas13 proteins are capable of both pre-crRNA processing and target RNA cleavage, which protect the host from phage attacks. Once bound to a target RNA, their non-specific RNase activity is activated. Cas13b has many distinctive features compared to the other Cas13 proteins, including the lack of significant sequence similarity, disparate crRNA repeat region, and double-sided protospacer flanking sequence (PFS)-dependent target RNA cleavage.
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