Evidence for multiple immunoglobulin genes

Antibody diversity

Three important aspects of immunoglobulin gene organization and structure have emerged from studies of cloned immunoglobulin kappa chain genes. (i) Multiple variable genes are encoded separately in the genome of both immunoglobulin-producing and uncommitted (embryonic) cells, thereby establishing the evolutionary base for generating immunoglobulin diversity. (ii) These genes exist as many small, closely related families (subgroups) that share close sequence homology largely within their own subgroup. (iii) Comparison of two cloned variable gene segments derived from a single subgroup reveals a feature of their structure that distinguishes them from fixed genes (that is, globin genes) and provides, through extensive surrounding sequence homology, a large target for intergenic recombination. This last observation suggests that a simple recombination mechanism may account for their genetic instability in both germ line and somatic cells.

Immunochemical isolation of gamma-globulin mRNA and estimation of immunoglobulin gene reiteration

The polyribosomes synthesizing gamma-globulin have been isolated by the achievement of specific precipitation using bentonite-treated anti-IgG antibody. The RNA extracted from the immunochemically precipitated polysomes was tested for its ability to direct the synthesis of proteins in a cell-free system. The specific gamma-globulin-synthesizing activity (cpm of gamma-globulin synthesized/microgram RNA) of this RNA was 10-fold greater than that from total polysomes. gamma-globulin mRNA (messenger RNA) isolated by immunoprecipitation was more than 89% pure with respect to contamination by other species of mRNA. The products synthesized by the cell-free system were also analyzed by sodium dodecyl sulphate(SDS)-polyacrylamide gel electrophoresis. This RNA has been hybridized with mouse myeloma DNA. The estimation of immunoglobulin gene reiteration was carried out using hybridization kinetics with consideration given to the DNA/RNA ratio since the estimation from the "half Cot value" is not accurate. The results suggest that in the mouse there are about 20 copies per subgroup of genes coding for the variable region of the H and L chains.

Quantitation of constant and variable region genes for mouse immunoglobulin lambda chains

We have synthesized and characterized cDNA complementary to purified mRNA derived from the lambda chain producing myeloma tumor, RPC-20. This cDNA is of sufficinet length to encode the constant region and a major portion of the variable region sequence of the lambda gene. In addition, the expected range of cross-hybridization of this lambda probe has been shown to extend to several different members of the closely related lambda subgroup, as well as to a member of the lambda subgroup represented by MOPC-315. Since there are a minimum of seven known members of the common lambda subgroup in addition to MOPC-315, these sequences, in accordance with the germ line hypothesis, must be represented by a minimum of eight variable region genes. Using the RPC-20 cDNA probe and hybridization kinetic analysis, this sequence was found to be represented as approximately two copies per haploid genome in DNA derived from a variety of k-and lambda-producing tumors and normal tissue. Inasmuch as the cross-hybridization range of the probe has been assessed and a minimum size of the lambda subgroup determined, this observation tends to rule out separate germ line genes corresponding to each individual lambda light chain variant. Certain reservations about these conclusions are discussed.

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.

Reiteration frequency of immunoglobulin light chain genes: further evidence for somatic generation of antibody diversity

Methods have been developed for preparing mouse immunoglobulin light chain mRNA of better than 90% purity. Hybridization of both lambda and kappa mRNAs to excess liver DNA yielded results compatible with gene reiteration frequencies of two to three. There was no evidence of hybridization of these highly purified mRNAs to reiterated DNA, and, in fact, the kinetics of hybridization were very similar to that of purified globin mRNA. Purified lambda mRNA from tumors producing structurally different lambda chains were used in competition hybridization experiments. An unlabeled lambda mRNA competed with another, labeled lambda mRNA to the same extent as homologous unlabeled lambda mRNA. That is, base sequence homology among lambda mRNAs is so high that any lambda mRNA should cross-hybridize with all germ line variable (Vlambda) genes at least for those V-regions which are represented among myelomas. From amino-acid sequence data, it is argued that there are probably more than 25 different lambda V regions. Hence it is concluded that the number of germ line genes is too small to account for the diversity of lambda chains. A similar conclusion is drawn for kappa chains.

Mouse immunoglobulin genes: studies on the reiteration frequency of light-chain genes by hybridisation procedures

The partially purified immunoglobulin light chain messenger RNA fraction from P3K (MOPC 21) mouse myeloma tissue-culture cells has been employed in hybridisation studies. Fragments of the messenger RNA were generated by alkali hydrolysis. 6-S fragments not containing poly(A) showed the characteristic biphasic hybridisation profile seen with the intact RNA fraction. 12-S and 6-S poly(A)-containing fragments, however, showed single transitions lacking the rapidly hybridising component. Complementary DNA copies of the intact messenger RNA fraction were prepared with RNA-dependent DNA polymerase and the DNA populations fractionated on acrylamide gels. Hybridisation experiments with complementary DNA fractions up to 800 bases in length showed annealing to single (or a few) genes. A rapidly hybridising component (about 200 copies) appears in the cDNA fraction containing the largest transcripts. We conclude that the kappa constant region gene and the MOPC 21 variable region gene are present as one or a few copies in the haploid genome and that the rapidly hybridising component is not due to variable region genes.

Messenger RNA metabolism of animal cells. Possible involvement of untranslated sequences and mRNA-associated proteins

The past several years have seen a virtual revolution in the study of eukaryotic mRNA. Among the notable recent achievements are the positive identification of mRNA precursors in HnRNA, the enumeration of the DNA sequences from which mRNA is transcribed, and the finding that mRNA in cultured cells is much more stable than was previously believed. One of most far-reaching discoveries has been the finding that mRNA in eukaryotes contains poly A. This discovery, aside from providing a powerful tool for mRNA isolation, has generated a large body of research into the properties and metabolism of poly A itself. In addition, the finding of a poly A-associated protein has given a renewed stimulus to the study of proteins associated with mRNA. This review is devoted to a discussion of these and related achievements, and some of their implications

Organization of immunoglobulin genes: reiteration frequency of the mouse kappa chain constant region gene

Hybridization kinetic analyses with synthetic DNA indicate that there are only two to three copies of the kappa constant region gene per haploid genome. This result lends weight to the argument that the immunoglobulin light chain is encoded by more than one continuous gene sequence.