Dual expression of λ genes in the MOPC-315 plasmacytoma

The expression of two distinct κ light chain immunoglobulins in the MPC-11 mouse myeloma is well established^1-4, the two protein products being apparently from RNA transcripts derived from separate, rearranged κ alleles in the MPC-11 genome^5-7. 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 λ_II 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 λ_II 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 Λ_II chain mRNA and another which directs the synthesis of a deleted form of λ_I protein. The nucleotide sequence of the deleted Λ_I, 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.