Activation-induced cytidine deaminase (AID) can target both DNA strands when the DNA is supercoiled.
The activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class-switch recombination of Ig genes. It has been shown that in vitro, AID protein deaminates C in single-stranded DNA or the coding-strand DNA that is being transcribed but not in double-stranded DNA. However, in vivo, both DNA strands are mutated equally during SHM. We show that AID efficiently deaminates C on both DNA strands of a supercoiled plasmid, acting preferentially on SHM hotspot motifs. However, this DNA is not targeted by AID when it is relaxed after treatment with topoisomerase I, and thus, supercoiling plays a crucial role for AID targeting to this DNA. Most of the mutations are in negatively supercoiled regions, suggesting a mechanism of AID targeting in vivo. During transcription the DNA sequences upstream of the elongating RNA polymerase are negatively supercoiled, and this transient change in DNA topology may allow AID to access both DNA strands.
Department of Molecular Genetics and Cell Biology, University of Chicago, 920 East 58th Street, Chicago, IL 60637, USA. hshen1@midway.uchicago.edu
Hong Ming
HM
Shen
Ursula
U
Storb
D001483:Base Sequence; D003562:Cytidine / Q000378:metabolism; D003564:Cytidine Deaminase; D043525:Cytosine Deaminase / Q000378:metabolism*; D004278:DNA, Superhelical / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D008969:Molecular Sequence Data; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D004278:DNA, Superhelical; D003562:Cytidine; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D043525:Cytosine Deaminase; D003564:Cytidine Deaminase