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

UV transcription mapping with recombinant DNA probes containing immunoglobulin kappa light chain mRNA sequences has been used to determine the size of the transcription unit coding for kappa light chain m RNA and to establish the arrangement of variable and constant regions in this transcription unit. 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 our previous pulse-labeling results which showed that the largest presumptive nuclear RNA precursor to kappa light chain mRNA is approximately 10 kb. In addition, the UV target size of cytoplasmic kappa mRNA is indistinguishable from the target size of constant region sequences in nuclear RNA. These results suggest that 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. Accordingly, it is proposed that 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.