A complete immunoglobulin gene is created by somatic recombination

Abstract

Using a pCRI plasmid containing an enzymatically synthesized, full-length DNA transcript of immunoglobulin λ chain mRNA as the hybridization probe in the Southern gel blotting experiments ( Southern, 1975), we identified three DNA fragments of 8.6, 4.8 and 3.5 kb in Eco RI-digested total DNA from BALB/c mouse embryos. A fourth fragment of 7.4 kb was found in addition to these three fragments in similarly digested total DNA from a λ chain-secreting myeloma (HOPC 2020). We have cloned the four DNA fragments in an EK-2 phage vector, λ WES and characterized them with respect to size, type of λ gene sequences contained and position of these sequences in the fragments, using agarose gel electrophoresis, the gel blotting technique and electron microscopic R loop mapping. The embryonic DNA clones Ig 99λ, Ig 25λ and Ig 13λ contain one copy each of V λl, C λI and V λlI sequences, respectively, while the myeloma DNA clone Ig 303λ contains one copy each of V λI, and C λI sequences that are separated by a 1.2 kb nontranslated DNA segment. Ig 25λ was also shown to contain a DNA segment of approximately 40 base pairs (bp) (J sequence) that lies 1.2 kb away from the Ca λI sequence and is homologous to the V-C junction region of a λ I mRNA. Heteroduplex analysis of the three λ I DNA clones revealed that Ig 303λ DNA is composed of two parts, one of which is entirely homologous to one end of Ig 99λ, and the other to one end of Ig 25λ DNA. The sequence arrangement observed in the cloned DNA is the same as that in the corresponding cellular DNA. This was shown by identifying certain restriction enzyme cleavage sites on the cloned DNAs and demonstrating the presence of these sites in the total cellular DNA by the gel blotting technique. The site of the homology switch is at the boundary of the V sequence and the 1.2 kb nontranslated DNA segment, and corresponds to the position of the J sequence on the Ig 25λ DNA. We consider the above experimental results the most direct evidence for somatic rearrangement in immunoglobulin genes. We discuss the significance of these findings for the origin of genes in the evolution of higher organisms and in cell differentiation.