Sequences of the joining region genes for immunoglobulin heavy chains and their role in generation of antibody diversity.

To assess the contribution to immunoglobulin heavy chain diversity made by recombination between variable region (VH) genes and joining region (JH) genes, we have determined the sequence of about 2000 nucleotides spanning the rearranged JH gene cluster associated with the VH gene expressed in plasmacytoma HPC76. The active VH76 gene has recombined with the second germ-line JH gene. The region we have studied contains two other JH genes, designated JH3 and JH4. No other JH gene was found within the region 1000 nucleotides downstream from JH4. Between JH3 and JH4 there is a pseudo-JH sequence with substantial homology to the authentic JH genes. The four JH genes whose sequences now are known can account for all known JH amino acid sequences. The JH genes are more divergent than the Jk genes and vary in length, encoding either 15 or 17 amino acid residues. Because JH regions comprise part of the third hypervariable region (HV3), combinatorial VH--JH joining substantially augments VH diversity. Moreover, a VH gene can recombine with each JH gene at several positions, and either one or two germ-line JH codons can be excised. This JH truncation markedly reduces the length of HV3 and hence must alter antigen-binding specificity. We have also determined the sequence of the JH4 region in two different gamma 2a mRNAs and have found that each has suffered a point mutation (aspartate to asparagine) which would alter the charge of the antigen-binding site.