Polysome Translational State during the Cell Cycle

HeLa cells were synchronized with a double thymidine block. Ribosomal subunits, monomers and polyribosomes have been quantitatively analysed at hourly intervals, during interphase, and every 15 min, during mitosis. This analysis was performed on linear 7–47% sucrose gradients.From the beginning of G1 up to the end of S phase, a certain equilibrium among ribosomal sub‐units, monomers and polyribosomes is maintained, while from the time of entering G2 to M the translation machinery appears to be mobilized in the sense of polysome formation. Under these conditions, the amount of polysomes per cell during the mitotic cycle is expressed by a bi‐phasic pattern showing pre‐ and post‐mitotic peaks with a falling‐off during S. The G1 peak, meanwhile, is much lower than the G2 peak.The incorporation of [3H]leucine into nascent polypeptide chains on polysomes, as well as into bulk cell proteins and into nuclear and cytoplasmic proteins considered separately, is also represented by a bi‐phasic curve which shows, however, a higher peak in G1 and a lower peak in G2, with two fallings‐off during S and M, respectively. Since between the G1 and the G2 amino acid pools there are not strong differenc