Biologically and Chemically Pure mRNA Coding for a Mouse Immunoglobulin L-Chain Prepared with the Aid of Antibodies and Immobilized Oligothymidine

The mRNA coding for a mouse immunoglobulin L-chain was prepared from MOPC-321 myeloma polysomes specifically precipitated with antibodies directed against L-chains, followed by chemical purification on oligo(dT)-cellulose. Biological purity (capacity to program the synthesis only of L-chain) was calculated to be >/=95%. This value was based on the estimation of contamination by non-L-chain mRNA activities that were present in large abundance in RNA preparations extracted from the total polysome population. A similar degree of purity was calculated from the extent of precipitation of myeloma and nonmyeloma polysomes with anti-L-chain and non-L-chain antibodies. Chemical purity (95%) was determined from the amount of rRNA in the mRNA preparation by scanning of appropriate gels. In a cell-free system, the purified mRNA directed the synthesis of two precursors heavier than L-chain by about 1300 and 4700 daltons. Cell-free products labeled with 10 (14)Caminoacids yielded 27 out of 28 expected L-chain tryptic peptides and four additional peptides. Most probably the latter were derived from extra pieces in the precursors, and the apparent loss of one peptide was due to modifications at the N-terminus. The main fraction of L-chain mRNA was composed of two species of about 420,000 and 450,000 daltons. These molecules are much larger than that required to code for a mature L-chain (calculated about 250,000). The additional nucleotide mass can be accounted for in part for the coding of the extra piece (about 50,000) and in part for the polyadenylate moiety.