Expression of IgD may use both DNA rearrangement and RNA splicing mechanisms
From a library of mouse sperm DNA, we have isolated two overlapping clones which contain the C 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 DNA. The C 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 δmRNA 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 Cµ and C 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.