The Organization and Diversity of Immunoglobulin Genes

We have used purified mouse immunoglobulin light chain mRNA and synthetic DNA which is complementary to it to assess the reiteration frequency of gene sequences corresponding to the κ constant region of the mouse immunoglobulin light chain. These studies indicate that the constant region sequence is represented only two to three times per haploid mouse genome, a finding that rules out a simple stringent germ line mechanism which would require the constant region sequence to be represented hundreds if not thousands of times. Hybridization studies involving ¹²⁵I-labeled myeloma light chain mRNA yield interesting results which may eventually permit us to distinguish between the remaining somatic mutation and recombinational germ line hypotheses. These results reveal a major component of relatively unique frequency and a minor component with a reiteration frequency of approximately 30 to 50 copies per haploid genome. As discussed, these results do not permit us to distinguish unambiguously between a germ line model and a type of somatic mutation model that permits germ line genes corresponding to each κ subgroup. The 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.