Abstract
Four major mRNA species of mouse sarcoma ascites cells, coding for polypeptides designated P65, P40, P36, and P21, occur predominantly as untranslated messenger ribonucleoprotein particles. Cloned cDNA probes were used to study their distribution in cytoplasmic extracts of these cells. A considerable portion of the mRNA molecules sedimented as small particles, whereas the rest was present in polyribosomes. In contrast, the actin mRNA was present almost exclusively in polyribosomes. Incubation of the ascites cells in culture medium, particularly after a starvation treatment, caused an enhancement in polypeptide chain initiation relative to elongation in these cells, as evidenced by a shift of ribosomes into the polyribosome fraction and by an increase in polyribosome size. Exposure of the cells to a low concentration of cycloheximide, an inhibitor of the elongation step, had a similar effect. The actin mRNA and the active P65, P40, P36, and P21 mRNA molecules were shifted to larger polyribosomes in the treated cells, but no shift of molecules from small particles to polyribosomes was observed. The incubation in culture also led to considerable increases in the proportion of P65 and P40 mRNA molecules in the untranslated state. The results indicate that the untranslated state cannot be attributed to poor initiation efficiency. It is suggested that a portion of the mRNA molecules is maintained in a repressed state and that mRNA repression may represent an important translation control process.