In the genome of a germ-line cell, the genetic information for an immunoglobulin polypeptide chain is contained in multiple gene segments scattered along a chromosome. During the development of bone marrow-derived lymphocytes, these gene segments are assembled by recombination which leads to the formation of a complete gene. In addition, mutations are somatically introduced at a high rate into the amino-terminal region. Both somatic recombination and mutation contribute greatly to an increase in the diversity of antibody synthesized by a single organism.

Somatic generation of antibody diversity

Hybridization of lambda-mRNAs to excess liver DNA yielded results compatible with gene reiteration frequencies of three or less. Purified mRNA from tumors producing structurally different lambda chains were used in competition hybridization experiments. An unlabeled lambda-mRNA competed with another, labelled mRNA to the same extent as homologous unlabelled lambda-mRNA. Mouse DNA was digested with Eco R-I restriction endonuclease and fractionated by gel electrophoresis. A DNA fragment carrying the V lambda-gene(s) was indentified in this digest. This fragment hybridized with lambda-mRNAs coding for two different lambdaV regions equally well. These results indicate that base sequence homology among lambda-mRNAs is so high that any lambda-mRNA should cross-hybridize with all or most of germ line V lambda genes. From amino acid sequence data, it is argued that there are probably more than 25 different lambdaV regions. Hence it is concluded that the number of germ line genes is too small to account for the diversity of lambda chains.

Somatic generation of antibody diversity.

Publisher Summary This chapter describes the origin and evolution of retroposons. It deals with two of them which operate on very different timescales: (1) RNA splicing (particularly messenger RNA splicing), which is the excision of transcribed noncoding sequences from RNA during normal gene expression and (2) retroposition, which is the insertion of reverse-transcribed sequences from RNA back into the genome during evolution. RNA splicing was the first and most surprising discovery of eukaryotic molecular genetics. Retroposons are a class of dispersed sequences in DNA, which appear to have arisen during evolution by a particular mechanism of inserting RNA sequences into chromosomal DNA (retroposition). They include processed mRNA pseudogenes, snRNA pseudogenes, the highly repeated ALU sequences, and a variety of other sequences. They are entirely distinct from transposons and retroviruses. The chapter explains the splicing of ribosomal RNA, mitochondrial RNA, and chloroplast RNA.

The Origin and Evolution of Retroposons

https://www.cell.com/cell/pdf/0092-8674(84)90444-6.pdf

Mouse T cell antigen receptor: Structure and organization of constant and joining gene segments encoding the β polypeptide

Article de synthese sur la comprehension et les concepts actuels de l'induction de la regulation et des mecanismes moleculaires de la commutation de classe des immunoglobulines

Immunoglobulin class switching: molecular and cellular analysis.

From a library of mouse sperm DNA, we have isolated two overlapping clones which contain the Cδ gene. One of these clones also contains the Cμ gene. The Cδ gene is separated from the Cμ membrane exons by approximately 2 kilobases (kb) of DAN. The Cδ gene was identified by (a) hybridization to poly(A)+RNA prepared from the IgD-producing rat plasma cell tumor IR731, and (b) homology of a translated nucleotide sequence to the amino acid sequence of the human δ chain. The Cδ gene spans 8 kb of DNA in the germ line. Plasmid subclones of the Cδ gene were used as probes in Southern and RNA blot experiments. RNA blot analysis of cytoplasmic poly(A)+RNA from IR731 and a μ+δ+ B-cell hybridoma revealed 1.6- and 2.7-kb δ mRNA species with different 3′ ends, which presumably encode the secreted and membrane-bound forms, respectively, of the δ chain. Southern blot analysis of DNA from two μ+δ+ lymphomas revealed that the Cδ gene is in the germ-line configuration in each case. Restriction map analysis of Cμ and Cδ genomic clones isolated from a library of normal μ+δ+ B-cell DNA also gave no evidence for DNA rearrangement in the region between the Cμ and Cδ genes. Taken together, these data suggest that IgD expression in μ+δ+ B cells does not involve a VH-to-Cδ DNA switch rearrangement. We propose that simultaneous expression of Cδ and Cδ with a single VH gene is mediated by two alternative routes of RNA processing of a primary nuclear transcript which contains the VH, Cμ, and Cδ genes. In contrast, analogous experiments with myeloma IR731 DNA revealed that the Cμ gene has been deleted from the myeloma DNA and that the Cδ gene has undergone DNA rearrangement, presumably including a switch recombination of the VH gene from the Cμ to the Cδ gene. These results indicate that two alternative mechanisms may be used in the expression of IgD molecules—RNA splicing in B cells and DNA rearrangement in plasma cells.

Expression of IgD may use both DNA rearrangement and RNA splicing mechanisms

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Expression of IgD may use both DNA rearrangement and RNA splicing mechanisms