The expression of two distinct κ light chain immunoglobulins in the MPC-11 mouse myeloma is well established1-4, the two protein products being apparently from RNA transcripts derived from separate, rearranged κ alleles in the MPC-11 genome5-7. Recently, the characterization of κ-related RNAs and protein products In several λ-producing myelomas has indicated that multiple expression of light chain RNAs is a common event in myelomas and other cells of the B-lymphocyte lineage5. These studies suggest that, although many light chain alleles may function to make RNA and protein in a given B-lymphocytic cell, only one complete, functional light chain is generally translated from the RNAs present in a single cell. The myeloma, MOPC-315, synthesizes and secretes an antibody which has an α heavy chain and a λII light chain8. The DNA of MOPC-315 either has no κ genes or has only a fragment of one, but it certainly has no κ genes in the embryonic configuration5. Rearrangement of its λ genes has been observed but the exact nature of the rearrangement is not known9. Because initial observations suggested that an immunoglobulin-related protein other than α and λII was present in MOPC-315 cells10, we undertook to derive molecular cDNA clones from the mRNA in MOPC-315 tumour cells. Analysis of the clones has now identified two λ chain mRNA species: a normal ΛII chain mRNA and another which directs the synthesis of a deleted form of λI protein. The nucleotide sequence of the deleted ΛI, mRNA shows that it resulted from a joining of the sequence encoding amino acid 31 of the variable region directly to the constant region coding sequence.
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