Fine structure and activity of discrete RAG-HMG complexes on V(D)J recombination signals

Two lymphoid cell-specific proteins, RAG-1 and RAG-2, initiate V(D)J recombination by introducing DNA breaks at recombination signal sequences (RSSs). Although the RAG proteins themselves bind and cleave DNA substrates containing either a 12-RSS or a 23-RSS, DNA-bending proteins HMG-1 and HMG-2 are known to promote these processes, particularly with 23-RSS substrates. Using in-gel cleavage assays and DNA footprinting techniques, I analyzed the catalytic activity and protein-DNA contacts in discrete 12-RSS and 23-RSS complexes containing the RAG proteins and either HMG-1 or HMG-2. I found that both the cleavage activity and the pattern of protein-DNA contacts in RAG-HMG complexes assembled on 12-RSS substrates closely resembled those obtained from analogous 12-RSS complexes lacking HMG protein. In contrast, 23-RSS complexes containing both RAG proteins and either HMG-1 or HMG-2 exhibited enhanced cleavage activity and displayed an altered distribution of cleavage products compared to 23-RSS complexes containing only RAG-1 and RAG-2. Moreover, HMG-dependent heptamer contacts in 23-RSS complexes were observed. The protein-DNA contacts in RAG-RSS-HMG complexes assembled on 12-RSS or 23-RSS substrates were strikingly similar at comparable positions, suggesting that the RAG proteins mediate HMG-dependent heptamer contacts in 23-RSS complexes. Results of ethylation interference experiments suggest that the HMG protein is positioned 5' of the nonamer in 23-RSS complexes, interacting largely with the side of the duplex opposite the one contacting the RAG proteins. Thus, HMG protein plays the dual role of bringing critical elements of the 23-RSS heptamer into the same phase as the 12-RSS to promote RAG binding and assisting in the catalysis of 23-RSS cleavage.