Three New Cs for CRISPR: Collateral, Communicate, Cooperate

The core feature of CRISPR-Cas systems is the use of RNA-guided Cas nucleases that use a short RNA to locate and cleave complementary nucleic acids. Recent studies uncovered features that add substantial complexity to this central mechanism. Upon crRNA-guided recognition of complementary nucleic acids, many CRISPR-Cas systems also degrade nontarget RNA or ssDNA. The RNA-guided recognition of target RNA molecules triggers the production of cyclic oligoadenylate; a secondary messenger that activates the nonspecific degradation of RNA during type III CRISPR-Cas immunity. Many inhibitors of Cas nucleases have been found in phage genomes, which require the cooperative infection of viruses to gradually suppress the CRISPR-Cas immune response