Fig. 3.
Genesis and processing of germline transcripts. Each constant region (CH) contains an individual transcriptional unit that produces noncoding germline transcripts (GLTs). Within each unit, transcription initiates upstream of the switch (S) regions, and is promoted by mitogen and/or cytokine-responsive elements (right box), with the exception of Cμ GLTs which are driven by the μ enhancer (Eμ) and are constitutively expressed (left box). Transcription proceeds through the intervening, or I-exon, intronic S regions and CH exons to produce a primary GLT. Splicing of the I-exon to the 5′ exon of the downstream CH gene creates a processed GLT, and an intronic S region lariat that can undergo debranching to become a linear S region transcript. Solid arrows indicate transcription start sites, dashed lines within transcripts indicate introns, solid lines within transcripts indicate exons. AID, activation-induced cytidine deaminase.