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Immunoglobulin gene expression requires that cells combine the correct genetic information by DNA rearrangement and RNA processing to produce mature, translatable mRNAs. The mechanisms that operate to produce IgD heavy chain (δ chain) mRNA are thought to be especially complex1–3. Like other immunoglobulins, IgD exists in membrane and secreted forms4,5. By analogy with results of studies of μ6–10and γ11,12gene expression, we expected that δ chains would also be encoded by two different mRNAs, containing common sequence information transcribed from constant region gene segments but different information transcribed from coding sequences located 3′ to the constant region gene segments. Instead, we identified13more than two δ mRNAs in normal mouse spleen and in two IgD-secreting plasmacytomas14, TEPC 1017 and TEPC 1033. Now we have used 32P-labelled DNA fragments prepared from a δ cDNA clone15and a δ genomic clone16to characterize by hybridization these δ RNAs, fractionated on methyl mercury hydroxide agarose gels. We find that normal mouse spleen contains a major 2.9-kilobase (kb) and a minor 2.1-kb RNA encoding membrane-bound δ chains (δm) and that TEPC 1017 and TEPC 1033 contain similar δmRNAs plus a 1.75-kb RNA encoding secreted δ chains (δm). We have also observed less abundant δ RNA species (2.65 and 3.2 kb). Different gene segments or combinations of segments are used in the 3′ termini of the multiple δ RNAs.

We have sequenced secreted and membrane terminal exons of the murine immunoglobulin δ chain. The putative transmembranal exon is strikingly similar to that of the μ chain and the internal cytoplasmic exon codes for exactly the same two amino acids, valine and lysine. A third potential exon was found between S and M exons that could code for yet another hydrophilic carboxyl terminus termed δX.

A mouse α-globin-related pseudogene (ψα30.5) completely lacks intervening sequences, and could not code for a functional globin polypeptide because of frameshifts. The widespread occurrence of globin pseudogenes in other species suggests that they are not ‘dead’ genes but may be important in controlling globin expression.

The epidermis of normal mice contains two distinct populations of dendritic cells derived from the bone marrow, Ia+Langerhans cells and Ia−cells that express the Thy-1 alloantigen1–5. The Thy-1-bearing dendritic epidermal cells (Thy-1+dEC) have a surface phenotype similar to that of very early T-lineage cells6–8, produce IL-2-like growth factors10and exhibit cytotoxicity which is not restricted by the major histocompatibility complex (MHC)10,11. The relationship of Thy-1+dEC to the T-cell lineage is unclear. Most T lymphocytes bear a receptor for antigen composed of an α chain and a β chain12–17associated with a nonpoly-morphic complex termed CD3 (T3)18–21. A minor population carries a receptor in which CD3 is associated with a γ/δ complex21–25. We have analysed clones of Thy-1+dEC for rearrangement and expression of the genes for the α-, β- and γ-chains of the T-cell receptor (TCR). They do not express α or β but do carry a γ/δ complex. Activation of the cells with Con A is associated with a rapid decrease in the steady-state level of γ-chain RNA. Because Thy-1+dEC resemble early stage T lymphocytes, down-regulation of TCR expression may reflect a necessary event during T cell differentiation.

All human and murine immunoglobulin heavy chain variable region (VH) genes contain the sequence ATGCAAAT approximately 70 nucleotides 5′ from the site of transcription initiation1,2. This octanucleotide, in reverse orientation, is also found in all light chain variable region (VL) genes1,2, and in the immunoglobulin heavy chain transcriptional enhancer2. Transfection studies have established that this octamer is involved in the lyraphoid-specific transcription of immunoglobulin genes2–6. Octamer-containing fragments have been reported to bind a factor present in nuclear extracts of human cell lines; however, identical binding activity was detected in both B lymphoid and non-lymphoid cells7. Here we establish that nuclear extracts from distinct cell types differ in their ability to interact with octamer-containing fragments. We have also detected a DNA-protein interaction that may be involved in the cell-type specificity of immunoglobulin expression, and we have determined that a sequence upstream of the octamer participates in an interaction with a nuclear protein(s).

Most T cells bear an antigen receptor that is a protein of a disulphide-linked heterodimer composed of an a chain and a β chain associated with the non-polymorphic CD3 (T3) complex1–7. A small subpopulation of thymic and peripheral T cells8–17, as well as Thy-1 +dendritic epidermal cells (dEC) (refs 18–21), express an alternative CD3-associated dimeric receptor composed of the product of the T-cell antigen receptor (TCR) γ gene22and a fourth chain, designated δ. Recently a new murine TCR constant-region gene, designated C x, has been cloned and proposed as a candidate for the C δgene23. We have previously demonstrated that murine Thy-1+dEC cell lines express a CD3-associated disulphide-linked heterodimer composed of a relative molecular mass M r41,000 (41K) γ chain and a 50K δ chain21. We have further analysed the receptor of one of these cloned dEC lines, 7-17.1, by endoglycosidase treatment of the isolated γ and δ chains. The γ chain was found to contain two N-linked oligosaccharide residues, consistent with the expression of a chain encoded by the V γ3and C γ1gene segments. The δ chain contains at least three N-linked oligosaccharides and has a core size of 38K. Northern blot analysis indicated the presence of abundant C xmessenger RNA in 7-17.1 cells. Immunoprecipitation with two antisera to peptides comprising distinct regions of the C xsequence indicates that the δ chain is encoded by the C xgene.