Evidence for somatic generation of antibody diversity

RNA preparations containing 70-80% mouse kappa-chain mRNA have been prepared. The remainder consists of many RNA species, each of which represents a small fraction of the total RNA. The kappa-chain mRNA preparation hybridizes with mouse liver DNA with bi-phasic kinetics, indicating that it consists of two fractions -"unique" and "reiterated." Competition hybridization experiments show that the homology among the unique fractions from different mRNAs is the same as the homology among the amino acid sequences of the corresponding kappa-chains. Hence, in addition to the C-region (constant-region) sequences, (most of) the V-region (variable-region) sequences are also derived from unique germ line genes. The reiterated fractions from different kappa-chain mRNAs show essentially complete homology with each other. This fraction seems to consist mostly of sequences which do not code for amino-acid sequences of the secreted polypeptide chain, i.e., the "external" section of the mRNA molecule. It is concluded that the number of germ line genes is too small to account for the observed diversity of antibody molecules.