Structural basis of CRISPR-Cas Type III prokaryotic defence systems

CRISPR loci and CRISPR-associated (Cas) genes encode an adaptive immune system that protects many bacterial and almost all archaea against invasive genetic elements from bacteriophages and plasmids. Several classes of CRISPR systems have been characterized, of which the type III CRISPR systems exhibit the most unique functions. Members of type III cleave both RNA and DNA not only through their corresponding effector complexes but also by CRISPR-Cas associated proteins activated by second messengers produced by those effector complexes. Furthermore, the recent discovery of second messenger degrading proteins called ring nucleases adds an extra regulatory layer to fine-tune these immunity systems. Here, we review the defense mechanisms that govern type III CRISPR interference immunity systems focusing on the structural information available.