DISTINCT DNA-SEQUENCE AND STRUCTURE REQUIREMENTS FOR THE 2 STEPS OF V(D)J RECOMBINATION SIGNAL CLEAVAGE

Cleavage of V(D)J recombination signals by purified RAG1 and RAG2 prot eins permits the dissection of DNA structure and sequence requirements , The two recognition elements of a signal (nonamer and heptamer) are used differently, and their cooperation depends on correct helical pha sing. The nonamer is most important for initial binding, while efficie nt nicking and hairpin formation require the heptamer sequence, Both n icking and hairpin formation are remarkably tolerant of variations in DNA structure, Certain flanking sequences inhibit hairpin formation, b ut this can be bypassed by base unpairing, and even a completely singl e-stranded signal sequence is well utilized, We suggest that DNA unpai ring around the signal-coding border is essential for the initiation o f V(D)J recombination.