Estimation of the number of genes coding for the constant part of the mouse immunoglobulin kappa light chain.

As previously shown, purified 14S RNA of mouse myeloma MOPC-41 forms a single peak on sucrose gradients and gel electrophoresis and codes for a single polypeptide chain, the immunoglobulin light chain produced by the same myeloma in vivo. This 14S mRNA was used for the enzymatic synthesis of DNA (cDNA) which is complementary to the RNA template. A DNA fraction was isolated which has an average size of 300 nucleotides. Kinetic studies of the hybridization of the 14S RNA with the cDNA indicate that about 40% of the RNA consists of a single RNA sequence. From the size of the cDNA fraction and from the direction of DNA synthesis, it can be concluded that the cDNA includes the sequence complementary to the constant region of light chain mRNA. This radioactive cDNA was used for DNA.DNA reannealing experiments with unlabeled DNA from mouse liver or myeloma tumor in 3 x 10(6)-fold excess. This allowed the determination of the number of copies in the mouse genome of those sequences represented in the cDNA. The data show no significant reiteration in either liver or myeloma DNA and suggest that the gene coding for the constant part of immunoglobins is present in the haploid genome in one to five copies. Furthermore, this gene is not "amplified" in nuclear DNA of myeloma plasmocytes.