Hypermutation at A-T base pairs: the A nucleotide replacement spectrum is affected by adjacent nucleotides and there is no reverse complementarity of sequences flanking mutated A and T nucleotides.

Hypermutation is thought to be a two-phase process. The first phase is via the action of activation-induced cytidine deaminase (AID), which deaminates C nucleotides in WRC motifs. This results in the RGYW/WRCY hot spot motifs for mutation from G and C observed in vivo. The resemblance between the hot spot for C mutations and the reverse complement of that for G mutations implies a process acting equally on both strands of DNA. The second phase of hypermutation generates mutations from A and T and exhibits strand bias, with more mutations from A than T. Although this does not concur with the idea of one mechanism acting equally on both strands, it has been suggested that the AT mutator also has a reversible motif; WA/TW. We show here that the motifs surrounding the different substitutions from A vary significantly; there is no single targeting motif for all A mutations. Sequence preferences associated with mutations from A more likely reflect an influence of adjacent nucleotides over what the A mutates "to." This influence tends toward "like" replacements: Purines (A or G) in the 5' position bias toward replacement by another purine (G), whereas replacement with pyrimidines (C or T) is more likely if the preceding base is also a pyrimidine. There is no reverse complementarity in these observations, in that similar influences of nucleotides adjacent to T are not seen. Hence, WA and TW should not be considered as reverse complement hot spot motifs for A and T mutations.