Some sequence similarities among cloned mouse DNA segments that code for lambda and kappa light chains of immunoglobulins.

Abstract

A comparison between the cloned mouse DNA segments that were found to code for the X and K light chains of immunoglobulins established that there were seven short nucleotide sequences, two of which matched 6 out of 7, two 7 out of 8, two 8 out of 9, and one 9 out of 10 bases; these sequences were located either at homologous amino acid positions or at positions displaced by four amino acids or less. They all occurred in the framework regions (FRs), five next to the complementarity-determining regions (CDRs). Three of these were unique and did not occur elsewhere in the immunoglobulin nucleotides sequenced thus far or in DNAs of phage 4X174, phage G4, or simian virus 40. Five could serve as sites of joining by recombination or insertion of CDR to FR segments, and the invariant tryptophan that is the first residue of the second FR might serve as a sixth. These sites are consistent with the minigene or insertional hypotheses for the generation of antibody diversity but could also serve as points of recognition for a mutator enzyme or could serve to limit somatic mutation to the CDRs.The vast amount of experimental data on amino acid sequences of immunoglobulin variable regions (V regions) (1) has provided a basis for trying to understand antibody diversity. From a statistical analysis of such sequences (2, 3) as they were being accumulated, the three complementarity-determining (hypervariable) regions or segments (CDRs) were recognized in the light (L) and in the heavy (H) chains and predicted to form the walls of the antibody combining site; this has been verified by x-ray crystallography in four different laboratories (refs. 4-7; see ref. 8). As a consequence of this analysis each immunoglobulin chain was divided into four framework regions (FRs) separated by the three CDRs as follows: FR1, CDR1, FR2, CDR2, FR3, CDR3, and FR4. It was recognized that a simple mechanism for assembling the chains could be the insertion of nucleotides coding for the CDRs into those coding for the FR segments to assemble a complete V-region gene (2, 3). When the various FR segments for each chain were arranged into sets (9), each set being made up of segments identical in sequence, it was noted that the members of a given FR1 set could be associated with different FR2 sets, FR3 sets, and FR4 sets. This independent assortment of FR sets led to the hypothesis (9) that the V-region gene is assembled from sets of germ line minigenes for the FR segments and inferentially from sets of minigenes for the CDRs. A minigene was defined as a DNA segment coding for a portion of the V region that shows some evidence of segregation as a functional unit independent of the rest of the DNA coding for the V region (10). It was further hypothesized that the minigenes were assembled somatically during differentiation. Because assortment was demonstrated only for FR segments, it would be independent of whether one or two CDR residues assorted with a given FR. The CDRs could not be examined for independent assortment because there were too few tha...

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