CRISPR Cas 1 nuclease, molecular model. -associated protein 1 (Cas1) is a metal-dependent nuclease enzyme. Cas1 and another nuclease, Cas2, form a complex critical for assembling the CRISPR repeat segments. The mechanism is still unknown. This surface model shows Cas1 as a single protein chain folded into two domains (green and blue) connected by a hinge (orange). The active site contains a 2+ metal ion (magenta) that is responsible for the DNA-specific endonuclease activity of the enzyme.