Updating the definition of V(D)J recombination signal sequences with high-throughput recombination assays

Abstract V(D)J recombination initiates the production of a diverse antigen receptor repertoire in developing B and T cells. Antigen receptor diversity is required for adaptive immunity in all jawed vertebrates. Recombination activating proteins, RAG1 and RAG2 (RAG1/2), catalyze V(D)J recombination by cleaving adjacent to recombination signal sequences (RSSs) that flank antigen receptor gene segments. Previous studies defined the consensus RSS as containing conserved heptamer and nonamer sequences separated by a less conserved 12 or 23 base-pair spacer sequence. However, many RSSs deviate from the consensus sequence, and the molecular mechanism for semi-selective V(D)J recombination specificity is unknown. Here, we developed a cell-based, high-throughput V(D)J recombination assay to evaluate RAG1/2 activity on thousands of RSSs in parallel. Specifically, we focused our study on the RSS heptamer and adjoining spacer region because it undergoes extensive conformational changes during RAG-mediated DNA cleavage. While the consensus heptamer sequence (CACAGTG) was marginally preferred, RAG1/2 was highly active on a wide range of non-consensus sequences. RAG1/2 generally selected purine/pyrimidine motifs instead of specific bases. Molecular dynamics simulations of RSSs showed that DNA sequences preferred by RAG1/2 had dynamic structural features that may accommodate heptamer unwinding in the RAG1/2 active site. Our results suggest RAG1/2 specificity for RSS heptamers is primarily dictated by dynamic DNA properties dependent on purine/pyrimidine pattern. Further investigation of RAG1/2 specificity will help elucidate the genetic instructions guiding V(D)J recombination to antigen receptor gene segments. National Institutes of Health (AI128137, AI15635); Oklahoma Center for Advancement in Science and Technology (OCAST) [HR21-142]; Presbyterian Health Foundation of Oklahoma City; National Institute of Allergy and Infectious Diseases Immunology training grant [T32 AI7633-16]