THE ORGANIZATION OF IMMUNOGLOBULIN GENES

In order to distinguish among various models which have been advanced to account for the diversity of antibody molecules, we must know how the constant and variable regions of immunoglobulins are represented in the somatic genome. For this purpose, we have purified mRNA corresponding to a mouse kappa immunoglobulin light chain from the myeloma tumor, MOPC-41. This mRNA directs the enzymatic synthesis of highly radioactive DNA (cDNA). This cDNA, which should correspond to the constant region of the kappa chain, was assessed for reiteration frequency using hybridization kinetic analysis and was found to be represented approximately three times per haploid genome . This result tends to rule out germ line hypotheses which require many copies of the constant region gene. It also requires the postulation of a recombinational mechanism to join constant and variable region sequences. Hybridization kinetic analyses designed to assess the entire light chain sequence (C and V regions) made use of 125I-MOPC-41 mRNA. These revealed a major component of relatively unique frequency and a minor (~ 20%) component with a reiteration frequency of approximately 30-50 copies per haploid genome. Careful analysis of the extent of hybridization of this mRNA to DNA prepared from several tumors and tissues, thermal profiles, and relevant competition studies, while sensitive, do not permit us to distinguish unambiguously between a germ line model and the type of somatic mutation model which permits germ line genes corresponding to each kappa subgroup. Our results do, however, clearly rule out the existence of thousands of variable region sequences so closely related to the MOPC-41 V-region as to permit extensive, stable cross hybridization.