Messenger RNA coding for the deleted heavy chain of mouse myeloma MOPC 47A immunoglobulin.

Mouse IgA 47A is an immunoglobulin variant whose heavy (alpha) chain has a molecular weight of 40,000 and lacks the entire CH3 domain. In order to understand better the mechanism by which the CH3 domain is lost, the 47A heavy chain mRNA has been partially purified. The mRNA migrated as a 17 S species on sucrose gradients and coded for the cell-free synthesis of only one serologically reactive protein with an apparent molecular weight of 42,000. Competitive inhibition of immunoprecipitation and tryptic peptide analysis of a cyanogen bromide fragment of the in vitro product revealed significant homology between the authentically secreted heavy chain and the respective cell-free synthesized, immunoprecipitated product. The heavy chain mRNA of 47A possessed a chain length of about 2200 nucleotides which is the same size as a normal alpha heavy chain mRNA, thus demonstrating that the protein deletion is not due to a large deletion at the mRNA level. The deletion in the 47A heavy chain may be due to a frameshift of a single nucleotide followed by premature chain termination.