Isolation of messenger RNA for an immunoglobulin kappa chain and enumeration of the genes for the constatn region of kappa chain in the mouse

The messenger RNA for an immunoglobulin light (kappa) chain was isolated from the mouse myeloma MOPC41 and shown to be almost twofold longer than necessary to code for its protein product. DNA complementary to the mRNA was synthesized with the RNA-directed DNA polymerase of Rous sarcoma virus. Although the individual chains of the cDNA² contained an average of only 270 nucleotides, the cDNA was heterogeneous in molecular weight, allowing the isolation of a fraction of the cDNA 620 nucleotides long. This large cDNA would be long enough to code for nearly all (95%) of the constant region if all the untranslated region of the mRNA were between the 3′ terminal poly(A) and the constant region. On the other hand, if all the untranslated region of the mRNA were at the 5′ terminus, this cDNA would code for 93% of the entire kappa chain.The specificity of nucleotide sequences in the cDNA was documented by molecular hybridization with both template RNA and RNA from various myelomas. The amount of hybridization obtained with myeloma RNA was approximately proportional to the amount of kappa chain protein produced by the various myeloma cells. In addition, there was no hybridization with RNA isolated from either BALB/c mouse liver or Escherichia coli.The genes for the constant region of the kappa chain were enumerated in the mouse genome by annealing cDNA to DNA from mouse liver and MOPC41 myeloma. The haploid genome of both tissues contained three to four genes for the constant region of kappa chain even when tested under conditions that would detect distantly related nucleotide sequences. The fact that there are only a few genes coding for the constant region of kappa chains implies that specialized genetic mechanisms are required for the generation of antibody diversity.