Activation-induced deaminase (AID) catalyses deamination of deoxycytidine to deoxyuridine
within immunoglobulin loci, triggering pathways of antibody diversifi cation that are
largely dependent on uracil-DNA glycosylase (uracil- N -glycolase [UNG]). Surprisingly
effi cient class switch recombination is restored to ung / B cells through retroviral
delivery of active-site mutants of UNG, stimulating discussion about the need for UNG ’ s
uracil-excision activity. In this study, however, we fi nd that even with the overexpression
achieved through retroviral delivery, switching is only mediated by UNG mutants that
retain detectable excision activity, with this switching being especially dependent on
MSH2. In contrast to their potentiation of switching, low-activity UNGs are relatively
ineffective in restoring transversion mutations at C:G pairs during hypermutation, or in
restoring gene conversion in stably transfected DT40 cells. The results indicate that UNG
does, indeed, act through uracil excision, but suggest that, in the presence of MSH2,
effi cient switch recombination requires base excision at only a small proportion of the
AID-generated uracils in the S region. Interestingly, enforced expression of thymine-DNA
glycosylase (which can excise U from U:G mispairs) does not (unlike enforced UNG or
SMUG1 expression) potentiate effi cient switching, which is consistent with a need either
for specifi c recruitment of the uracil-excision enzyme or for it to be active on singlestranded
DNA.