C-terminal deletion of AID uncouples class switch recombination from somatic hypermutation and gene conversion.

Class-switch recombination (CSR), somatic hypermutation (SHM), and antibody gene conversion are distinct DNA modification reactions, but all are initiated by activation-induced cytidine deaminase (AID), an enzyme that deaminates cytidine residues in single-stranded DNA. Here we describe a mutant form of AID that catalyzes SHM and gene conversion but not CSR. When expressed in E. coli, AID(delta189-198) is more active in catalyzing cytidine deamination than wild-type AID. AID(delta189-198) also promotes high levels of gene conversion and SHM when expressed in eukaryotic cells, but fails to induce CSR. These results underscore an essential role for the C-terminal domain of AID in CSR that is independent of its cytidine deaminase activity and that is not required for either gene conversion or SHM.