Fig. 1

Architecture and mechanism of CRISPR-Cas13 systems. Three main stages constitute the CRISPR-Cas13 immune response: adaptation, expression and interference. During the adaptation stage, a complex of Cas proteins binds the invading genome, which is shown as an RNA virus. The bound part of the target RNA is cleaved out and is inserted into the CRISPR array of the prokaryotic genome as a new spacer through a reverse transcriptase. The expression stage involves the transcription of the CRISPR array as a large, single transcript and this pre-crRNA is processed into a mature crRNA containing a target spacer and a flanking repeat. The mechanisms and components involved in the pre-crRNA processing of CRISPR-Cas13 systems have not been experimentally resolved yet. At the last stage of the immune response, the interference stage utilizes the crRNA as a guide to recognize invading genomes based on sequence complementarity, recruiting the complex of Cas proteins. The Cas13a/b/d proteins have two higher eukaryotes and prokaryotes nucleotide-binding (HEPN) domains of RNase activity, which cleave the target sequence and inactivate the RNA virus