An immunoglobulin heavy chain variable region gene is generated from three segments of DNA: V H, D and J H

Abstract

We have determined the sequences of separate germline genetic elements which encode two parts of a mouse immunoglobulin heavy chain variable region. These elements, termed gene segments, are heavy chain counterparts of the variable (V) and joining (J) gene segments of immunoglobulin light chains. The V H gene segment encodes amino acids 1–101 and the J H gene segment encodes amino acids 107–123 of the S107 phosphorylcholine-binding V H region. This J H gene segment and two other J H gene segments are located 5′ to the μ constant region gene (C μ) in germline DNA. We have also determined the sequence of a rearranged V H gene encoding a complete V H region, M603, which is closely related to S107. In addition, we have partially determined the V H coding sequences of the S107 and M167 heavy chain mRNAs. By comparing these sequences to the germline gene segments, we conclude that the germline V H and J H gene segments do not contain at least 13 nucleotides which are present in the rearranged V H genes. In S107, these nucleotides encode amino acids 102–106, which form part of the third hypervariable region and consequently influence the antigen-binding specificity of the immunoglobulin molecule. This portion of the variable region may be encoded by a separate germline gene segment which can be joined to the V H and J H gene segments. We term this postulated genetic element the D gene segment, referring to its role in the generation of heavy chain diversity. Essentially the same noncoding sequences are found 3′ to the V H gene segment and as inverse complements 5′ to two J H gene segments. These are the same conserved nucleotides previously found adjacent to light chain V and J gene segments. Each conserved sequence consists of blocks of seven and ten conserved nucleotides which are separated by a spacer of either 11 or 22 nonconserved nucleotides. The highly conserved spacing, corresponding to one or two turns of the DNA helix, maintains precise spatial orientations between blocks of conserved nucleotides. Gene segments which can join to one another (V κ and J κ, for example) always have spacers of different lengths. Based on these observations, we propose a model for variable region gene rearrangement mediated by proteins which recognize the same conserved sequences adjacent to both light and heavy chain immunoglobulin gene segments.