A high production rate of translatable IgG mRNA accounts for the amplified synthesis of IgG in myeloma cells.

The aim of the present work was to determine whether the accumulation of Ig mRNA in myeloma cells is due to a high rate of production or to a high stability of these molecules. Specific mRNAs for the light and heavy polypeptide chains of IgG were isolated from the murine MPC-11 myeloma tumor cells by immune precipitation of polysomes which synthesize these chains. It was found that the immune-precipitated polysomes were enriched 10--30-fold in the gamma and chi mRNA sequences respectively. In the wheat germ cell-free system the chi mRNA preparation was translated mainly into three polypeptides of Mr 25 000, 18 000, and 15 000. The method of immune precipitation of polysomes was also used to characterize three variant clones of MPC-11 myeloma. It was found that little if any gamma-chain polysomes are present in the L-chain producer and non-producer clones, while a substantial amount of chi-chain polysomes was present in the non-producer clone. This may be due to the presence in the non-producer cells of the constant region chi-chain fragment. In order to determine the relative synthesis rate of chi and gamma mRNAs, pulse-labelled polysomes were immune precipitated using antibodies to chi and gamma chains. It was found that chi and gamma mRNA molecules are produced at a very high relative rate each accounting for 10--15% of the total labeled mRNA after 1 h of labeling. These values are higher than the steady-state pool size of chi and gamma mRNA, which was 5--6%, and indicates that the half-life of these molecules is not unusually high. It is concluded that the amplified synthesis of immunoglobulin chains in myeloma cells is mainly due to a high rate of production of translatable chi and gamma mRNAs.