Characterization of light chain and light chain constant region fragment mRNAs in MPC 11 mouse myeloma cells and variants.

Cultured MPC 11 mouse myeloma cells synthesize not only gamma2b heavy and kappa light chains but also a carboxyl terminal (constant region) fragment of kappa light chain. In vitro translational analysis of total cytoplasmic and microsomal RNA indicates that these cells contain RNA which directs synthesis of both a light chain precursor and a light chain fragment precursor. Variant clones which do not synthesize either heavy or light chains continue to synthesize the light chain fragment. One such "nonproducing" variant was studied in detail. It does not contain translatable mRNA for the intact light chain but does contain RNA which is translated into the light chain fragment precursor. Nucleic acid hybridization analysis with a cDNA probe specific for the constant region of kappa light chains revealed that microsomal RNA from the wild-type cell contains both 14S and a 10S species of kappa specific RNA, whereas the variant contains only the 10S species. Translational analysis of these same RNAs indicates that the 14S species codes for the light chain precursor, while the 10S RNA codes for the light chain fragment precursor.