AbstractProtein synthesis in differentiated MOPC‐21 and MPC‐11 mouse myeloma cells was studied to determine the basis for the differences in the temperature and actinomycin D sensitivity of translation between non‐differentiated mouse L‐cells and differentiated rabbit reticulocytes. The temperature dependence of total protein synthesis was similar to that of L‐cells and reticulocytes, being biphasic in Arrhenius plots with apparent activation energies of approximately 25 and 42 kcal/mol, above and below 25°C. The dependence of the secretion process was different since it was not biphasic, having a single activation energy of about 22 kcal/mol. Myeloma polysomes were like L‐cell polysomes in their response to lower temperature and reached a minimum level of 50% at 15°C. This response was also found for the specific polysomes synthesizing the IgG H‐ and L‐chains. In the presence of actinomycin D, myeloma polysomes declined exponentially with a half‐life of ∼6 hours. These two L‐cell‐like responses were not found in reticulocytes. Translation of both the IgG mRNAs and the non‐IgG mRNAs was reduced by lower temperatures and actinomycin D, even though the L‐chain mRNA was slightly more resistant, suggesting that this mRNA is slightly more efficient. The results of these experiments suggest that the translational differences between L‐cells and reticulocytes are not mRNA dependent
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