Nuclear RNA precursors in the processing pathway to MOPC 21 κ light chain messenger RNA

Abstract

We have carried out pulse-chase and u.v. transcription mapping experiments which establish that immunoglobulin κ light chain messenger RNA in MOPC 21 myeloma cells is derived from a 10.6 × 103 base nuclear RNA precursor. This large nuclear RNA precursor is polyadenylated and appears to be processed into other polyadenylated κ-specific nuclear RNA species at 7.8, 5.6 and 4.8 × 103 bases. These are shown to be obligatory precursors to κ mRNA by glucosamine-mediated pulse-chase experiments in which virtually all of the κ sequences in these large nuclear RNAs are converted into κ mRNA. This kinetic evidence constitutes formal proof of the precursor-product relationship between these nuclear RNAs and cytoplasmic κ mRNA. In addition, the κ nuclear RNAs all show identical u.v. target sizes as expected for processing intermediates derived from the 10 × 103 base κ transcription unit. The major κ nuclear RNA intermediate at 4.8 × 103 bases appears to contain the intervening sequence separating the variable (V) and constant (C) regions in the active MOPC 21 κ gene. This 4.8 × 103 base species is shown directly to give rise to κ mRNA. Accordingly, we propose that V → C joining is the final splicing step in the nuclear RNA processing pathway for κ light chain mRNA.