Figure 2. Proposed model for RNA-dependent targeting of AID during class switch recombination.
Upon B-cell activation, germline transcription is initiated from a cytokine-inducible promoter (P) and primary germline transcripts are generated from the I-S-C H sequences, which encode the I-exon, switch (S) region, and constant coding exons (C H). These transcripts are spliced to form a mature non-coding, germline transcript and an intronic S lariat. The latter is further processed by the debranching enzyme DBR1 to form a linear S-region transcript. Linear S transcripts fold into G-quadruplex RNA, which is bound by AID. The complex of S-RNA and AID is guided to transcribed S-region DNA as a result of the complimentary between the S-RNA and the transcribed S region. AID, activation-induced cytidine daminase; DBR1, debranching RNA lariats 1.