Variable and constant regions are separated in the 10-kbase transcription unit coding for immunoglobulin kappa light chains.

Abstract

UV transcription mapping with recombinant DNA probes containing immunoglobulin .kappa. light chain mRNA sequences was used to determine the size of the transcription unit coding for .kappa. light chain mRNA and to establish the arrangement of variable and constant regions in this transcription unit in mouse myeloma cells. In relation to ribosomal RNA standards, the transcription of .kappa. light chain constant region sequences into nuclear RNA exhibits a UV target size of 9.6 kbases (kb). The .kappa. light chain variable region exhibits a UV target size of 7.6 kb indicating that it is separated by approximately 2.0 kb from the constant region in the .kappa. light chain transcription unit. The size of the primary transcript (i.e., the direct, unprocessed RNA product of transcription) predicted from the constant region target size concurs with previous pulse-labeling results which showed that the largest presumptive nuclear RNA precursor to .kappa. light chain mRNA is approximately 10 kb. The UV target size of cytoplasmic .kappa. mRNA is indistinguishable from the target size of constant region sequences in nuclear RNA. The .kappa. light chain transcription unit is copied directly into a 10 kb nuclear RNA precursor in which the .kappa. variable and constant regions are separated by approximately 2 kb. The joining of immunoglobulin .kappa. light chain variable and constant regions occurs in the post-transcriptional processing of this large nuclear RNA precursor into .kappa. light chain mRNA.