Fig. 1

CRISPR systems and their components encoded in viral genomes. A Organization of the CRISPR loci carried by viruses. Genes are shown by block arrows, roughly to scale. CRISPR repeats are shown by rectangles, and spacers are shown by diamonds; spacers with detectable matches are shown with colors, and spacers without matches are shown in grey. Cas genes are denoted by their systematic names, with some legacy names shown in grey. CEE, circular extrachromosomal element; HD, nuclease of HD family (named for its catalytic dyad); HP; hypothetical protein; PLE, phage-like element; TerS (terminase small subunit), TerL (terminase large subunit) and phage minor tail protein U, essential phage proteins; tracrRNA, transactivating CRISPR RNA. For the phage-encoded Cas9c protein, inactivation of the three catalytic motifs of the RuvC-like nuclease is indicated. B Competition between two archaeal viruses mediated by viral CRISPR mini-arrays; Spacers with matching protospacers in the respective viral genomes are shown in black and maroon, and spacers without matches are shown in grey. SPV1, SPV2, Saccharolobus portogloboviruses 1 and 2, two closely related, non-lytic archaeal viruses [14]. The other designations are as in A. C Inhibition of a host CRISPR systems by virus-encoded RNA anti-CRISPR. SRU, single repeat unit. Other designations are as in A. D General schematic of the functions of virus-encoded CRISPR systems and CRISPR components