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Nature. 2002 Jul 4;418(6893):99-103.

AID mutates E. coli suggesting a DNA deamination mechanism for antibody diversification.

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Medical Research Council Laboratory of Molecular Biology, Cambridge, UK.


After gene rearrangement, immunoglobulin variable genes are diversified by somatic hypermutation or gene conversion, whereas the constant region is altered by class-switch recombination. All three processes depend on activation-induced cytidine deaminase (AID), a B-cell-specific protein that has been proposed (because of sequence homology) to function by RNA editing. But indications that the three gene diversification processes might be initiated by a common type of DNA lesion, together with the proposal that there is a first phase of hypermutation that targets dC/dG, suggested to us that AID may function directly at dC/dG pairs. Here we show that expression of AID in Escherichia coli gives a mutator phenotype that yields nucleotide transitions at dC/dG in a context-dependent manner. Mutation triggered by AID is enhanced by a deficiency of uracil-DNA glycosylase, which indicates that AID functions by deaminating dC residues in DNA. We propose that diversification of functional immunoglobulin genes is triggered by AID-mediated deamination of dC residues in the immunoglobulin locus with the outcome--that is, hypermutation phases 1 and 2, gene conversion or switch recombination--dependent on the way in which the initiating dU/dG lesion is resolved.

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