Absence of control of poly(A)(+) messenger RNA translation in growth-stimulated mouse 3T6 fibroblasts.

To better understand the mechanism by which the rate of protein synthesis is regulated in growth-stimulated mouse 3T6 fibroblasts, we have determined the proportion of cytoplasmic poly(A)(+) mRNA that is associated with polysomes. We found that this proportion is nearly the same (70-80%)in resting, serum-stimulatd, or exponentially growing cells. This indicates that the increase in the rate of protein synthesis following serum stimulation of 3T6 cells is regulated primarily by the increase in mRNA content rather than by an increase in the fraction of total mRNA engaged in protein synthesis. We also show that in detergent-lysed 3T6 cells, up to half of the subpolysomal poly(A)(+) mRNA is of mitochondrial origin. This conclusion is based on analysis of the size of the mRNA and its poly(A), its subcellular location and the inhibition of the labeling of this mRNA by ethidium bromide. The physical properties of subpolysomal mRNA of nuclear origin are similar to those of polysomal mRNA.