Abstract 2077 The Cas4 C-terminal helix biases spacer acquisition toward foreign DNA

CRISPR-Cas systems provide adaptive immunity in bacteria and archaea by creating a genetic memory of past infections from mobile genetic elements. Many DNA-targeting systems require the Cas4-Cas1-Cas2 complex to effectively select, process, and integrate prespacers into the CRISPR loci. We previously solved a cryo-EM structure of Cas4-Cas1-Cas2 in complex with a half-site integration (HSI) intermediate from a type I-C system that revealed how Cas4 interacts with the prespacer and CRISPR array. One notable interaction was the C-terminal helix of Cas4 with the repeat DNA of the HSI.