Synapsis of Recombination Signal Sequences Located in cis and DNA Underwinding in V(D)J Recombination

Abstract

V(D)J recombination requires binding and synapsis of a complementary (12/23) pair of recombination signal sequences (RSSs) by the RAG1 and RAG2 proteins, aided by a high-mobility group protein, HMG1 or HMG2. Double-strand DNA cleavage within this synaptic, or paired, complex is thought to involve DNA distortion or melting near the site of cleavage. Although V(D)J recombination normally occurs between RSSs located on the same DNA molecule (in cis), all previous studies that directly assessed RSS synapsis were performed with the two DNA substrates in trans. To overcome this limitation, we have developed a facilitated circularization assay using DNA substrates of reduced length to assess synapsis of RSSs in cis. We show that a 12/23 pair of RSSs is the preferred substrate for synapsis of cis RSSs and that the efficiency of pairing is dependent upon RAG1-RAG2 stoichiometry. Synapsis in cis occurs rapidly and is kinetically favored over synapsis of RSSs located in trans. This experimental system also allowed the generation of underwound DNA substrates containing pairs of RSSs in cis. Importantly, we found that the RAG proteins cleave such substrates substantially more efficiently than relaxed substrates and that underwinding may enhance RSS synapsis as well as RAG1/2-mediated catalysis. The energy stored in such underwound substrates may be used in the generation of DNA distortion and/or protein conformational changes needed for synapsis and cleavage. We propose that this unwinding is uniquely sensed during synapsis of an appropriate 12/23 pair of RSSs.

We thank James C. Wang (Harvard University) for his generous gift of purified E. coli topoisomerase I. We also thank Yuk-Ching Tse-Dinh (New York Medical College) for providing us with AS 17 E. coli for expressing E. coli topoisomerase I. Discussions with Octavian Henegariu, Nigel Grindley, Sebastian Fugmann, and Chia-Lun Tsai were particularly helpful during the course of this work.

This work was supported by grant AI32524 to D.G.S. from the National Institutes of Health. M.C. is a postdoctoral associate, and D.G.S. is an investigator of the Howard Hughes Medical Institute.