Effect of reduced temperatures on protein synthesis in mouse L cells

The rate of incorporation of leucine into protein, the rate of polypeptide elongation and termination, and the relative quantity and size of polysomes were analyzed in mouse L cells grown in suspension culture at various temperatures between 0 degrees C and 36 degrees C. Between 10 degrees C and 36 degrees C protein synthesis exhibited two different apparent activation energies (39 kcal/mole, 10-25 degrees C; 14 kcal/mole, 25-36 degrees C), whereas elongation and termination had only one (16 kcal/mole). Below 36 degrees C, the polysome level and size decreased, reaching a minimum of 30% of the control 36 degrees C values at 10 degrees C; below 10 degrees C the level increased again back to control values at 0 degrees C. The polysome decline was time dependent, requiring about 5 hr to reach the equilibrium value. This decline is completely reversible within 60 min, even in the presence of 4 mug/ml of actinomycin D, and even after 15 hr of incubation at the lower temperature. The results suggest that polypeptide initiation is rate limiting, particularly below 25 degrees C; whereas above this temperature, elongation or perhaps some other process may be limiting. These results are quite different from those obtained for E. coli and rabbit reticulocyte protein synthesis.