Details of immunoglobulin gene rearrangement

A) The heptamer and nonamer recombination signal sequences. The 3'-flanking sequences of Vx genes have the palindromic heptamer recombination signal sequence, a 12-bp spacer, and the AT-rich nonamer signal sequence. The 5'-flanking region of Jx genes contain the sequences in the reverse order and separated by a 23-bp spacer. The immunoglobulin gene recombination mechanism only operates when one flanking sequence has a 12-bp spacer and the other, a 23-bp spacer (the 12- to 23-bp rule). The flanking sequences of lambda gene segments are identical but transposed. VH genes have 3' 23-bp spacers, D genes are flanked both 5' and 3' with 12-bp spacers, and JH genes have 5' 23-bp spacers.
B) Genetic products of rearrangement. Rearrangement proceeds by deletion of intervening DNA. The two gene segments are joined, as are the two signal sequences in a separate circular piece of DNA. The excised circular DNA contains all of the sequence which stood between the gene segments before rearrangement. The arrow within the gene segments indicates the transcriptional orientation of the gene segment (the direction taken by RNA polymerase when synthesizing mRNA). The rearrangement break points are indicated by arrows marked "X."
C) Consequences of imprecise gene segment joining. Ig gene segments need not always be spliced together at the same positions within their coding sequences. As long as the reading frame is maintained, a potentially functional protein may be generated. In the example here, four different in-frame splices are possible. Splicing at each position creates a codon for a different amino acid at the union of the two gene segments. Additional diversity may be generated by the random addition of nucleotides forming the N regions between the gene segments during splicing.