Use of 125I in fingerprinting RNA

No detectable reiteration of genes coding for mouse MOPC 41 immunoglobulin light-chain mRNA

RNA fractions rich in immunoglobulin light (L)-chain mRNA were isolated from mouse myeloma MOPC 41 by procedures previously described, and chemically labeled with 125I. These RNA fractions were hybridized with MOPC 41 DNA under conditions of DNA excess. Hybridization conditions were chosen under which the entire sequence of the L-chain mRNA probe, thus including the variable region, remains available for hybridization throughout the reaction. The hybridization (C0t) curve showed double transition kinetics, with one component corresponding to about 250 gene copies and the other to about two to four copies. In contrast, when MOPC 41 L-chain mRNA was further purified as a single band by gel elecptrophoresis in 99% formamide, the hybridization curve showed only a single transition, corresponding to about two to four genes, with the disappearance of the "reiterated" component. That component resulted therefore from contaminating RNA species. The data indicate that no reiteration can be detected by RNase or by hydroxylapatite for the genes corresponding to the entire sequence of MOPC 41 L-chain mRNA, including the untranslated segments, within the limits of detectability of short reiterated segments. It thus appears that there is only one or very few genes corresponding to the 41 L-chain variable region "subgroup" in MOPC 41 DNA. The possibility that the variable genes of plasmocytes might result frm a combination of several nonreiterated germline genes is discussed.

The selective iodination of yeast phenylalanine transfer RNA with 125-I

Yeast tRNA-Phe has been labelled with 125-I under conditions which conserve the tertiary structure. Significant labelling was only found to occur on specific cytidines in single stranded regions, while other cytidines in single stranded regions and all those in the double stranded region underwent iodination to a very small extent. The pattern obtained from iodine labelling satisfies the conformation of a model recently proposed for this tRNA.

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.