Cas1–Cas2 complex formation mediates spacer acquisition during CRISPR–Cas adaptive immunity

The CRISPR–Cas system mediates immunity to foreign DNA sequences that are integrated as spacers between repeats in the CRISPR locus. Work from Doudna and colleagues shows that nucleases Cas1 and Cas2 form a stable complex that recognizes the CRISPR leader-repeat sequence, thus determining the site of integration. The initial stage of CRISPR–Cas immunity involves the integration of foreign DNA spacer segments into the host genomic CRISPR locus. The nucleases Cas1 and Cas2 are the only proteins conserved among all CRISPR–Cas systems, yet the molecular functions of these proteins during immunity are unknown. Here we show that Cas1 and Cas2 from Escherichia coli form a stable complex that is essential for spacer acquisition and determine the 2.3-Å-resolution crystal structure of the Cas1–Cas2 complex. Mutations that perturb Cas1–Cas2 complex formation disrupt CRISPR DNA recognition and spacer acquisition in vivo. Active site mutants of Cas2, unlike those of Cas1, can still acquire new spacers, thus indicating a nonenzymatic role of Cas2 during immunity. These results reveal the universal roles of Cas1 and Cas2 and suggest a mechanism by which Cas1–Cas2 complexes specify sites of CRISPR spacer integration.