FIGURE 23-16 Mechanism of rearrangement of immunoglobulin gene segments via deletional joining. (a) Location of the DNA elements involved in somatic recombination of immunoglobulin gene segments at the light-chain locus (top) and at the heavy-chain locus (bottom). D segments are present in the heavy-chain, but not the light-chain, locus. At the 3′ end of all V gene segments is a conserved recombination signal sequence (RSS) composed of a heptamer, a 12-bp spacer, and a nonamer. Each of the J or D segments with which a V can recombine possesses at its 5′ end a similar RSS with a 23-bp spacer. The nonamer and heptamer sequences at the 5′ end of J or D are complementary and antiparallel to those found at the 3′ end of each V when read on the same (top) strand. The RSSs that flank the D segments have spacers of identical length, preventing the formation of D to D rearrangements. (b) Hypothetical model of how two coding regions to be joined may be arranged spatially, stabilized by the RAG1 and RAG2 recombinase complex. Both strands of the DNA are shown. (c) Events in the joining of V to J (light chain) or to DJ (heavy chain) coding regions. The germ-line DNA (step 1) is folded, bringing the segments to be joined close together, and the RAG1/RAG2 complex makes single-stranded cuts at the boundaries between the coding sequences and RSSs (step 2). The free 3′ –OH groups attack the complementary strands, creating a covalently closed hairpin at each coding end and a clean double-stranded break at each boundary with an RSS (step 3). The hairpins are opened, either symmetrically (step 4), as shown for the J (light chain) or DJ (heavy chain) segment, or asymmetrically (step 5), as shown for the V segment. For D to J and V to DJ rearrangements in the heavy-chain locus, terminal deoxynucleotidyl transferase adds nucleotides in a template-independent manner to opened hairpins (step 6, right), generating an overhang (yellow) of unpaired nucleotides of random sequence (N-region); asymmetric opening automatically creates a palindromic overhang (step 6, left). The unpaired overhangs at the ends of both the V and J (light chain) or DJ (heavy chain) coding regions are filled in by DNA polymerase (step 7) or may be excised by an exonuclease. DNA ligase IV joins the two segments generated from the V and J coding regions (step 8). N-region addition does not take place for V to J (light chain) rearrangements. See text for additional discussion.