Somatic hypermutation (SHM) drives the genetic diversity of Ig genes in activated B cells and supports the generation of Abs with increased affinity for Ag. SHM is targeted to Ig genes by their enhancers (diversification activators [DIVACs]), but how the enhancers mediate this activity is unknown. We show using chicken DT40 B cells that highly active DIVACs increase the phosphorylation of RNA polymerase II (Pol II) and Pol II occupancy in the mutating gene with little or no accompanying increase in elongation-competent Pol II or production of full-length transcripts, indicating accumulation of stalled Pol II. DIVAC has similar effect also in human Ramos Burkitt lymphoma cells. The DIVAC-induced stalling is weakly associated with an increase in the detection of ssDNA bubbles in the mutating target gene. We did not find evidence for antisense transcription, or that DIVAC functions by altering levels of H3K27ac or the histone variant H3.3 in the mutating gene. These findings argue for a connection between Pol II stalling and cis-acting targeting elements in the context of SHM and thus define a mechanistic basis for locus-specific targeting of SHM in the genome. Our results suggest that DIVAC elements render the target gene a suitable platform for AID-mediated mutation without a requirement for increasing transcriptional output.

Department of Genetics Harvard University Boston MA

Department of Immunobiology Yale School of Medicine New Haven CT; and

Howard Hughes Medical Institute Program in Cellular and Molecular Medicine Boston Children's Hospital Boston MA

Institute of Molecular Genetics Academy of Sciences of the Czech Republic Praha Czech Republic

The Azrieli Faculty of Medicine Bar Ilan University Safed Israel

Unit of Infections and Immunity Institute of Biomedicine University of Turku Turku Finland

Unit of Infections and Immunity Institute of Biomedicine University of Turku Turku Finland;

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Transcription elongation factor ELOF1 is required for efficient somatic hypermutation and class switch recombination

The SV40 virus enhancer functions as a somatic hypermutation-targeting element with potential tumorigenic activity