Cas9 is a bacterial immune protein that destroys invading DNA. A guide RNA holds the address, and the protein holds two nuclease domains that cut the two DNA strands. The guide RNA loads first and swings the two protein lobes into a channel that accepts DNA. The address sits in the RNA, not in the protein, so one enzyme reaches any sequence.
- Where
- Cytoplasm of bacteria with a type II CRISPR system. Laboratories now run it in nearly every cell type as an editing tool.
- Power
- None for targeting. Cas9 spends no ATP. Cleavage needs Mg2+ and the free energy of RNA-DNA base pairing.