Class 2 type VI-A CRISPR-associated RNA-guided ribonuclease Cas13a
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. Within the Cas13a (also called C2c2) subfamily, the active site is functionally diverse in terms of both nucleotide cleavage preference and turnover efficiency. There are two distinct types of Cas13a enzymes, based on their cleavage preference: adenosine (A) cleaving or uridine (U) cleaving.