Tracer kinetic studies have revealed the existence of a variable pattern of 3H-amino acid incorporation into amoeba proteins during the early G2 phase of the cell cycle. Two peaks of incorporation of [3H]leucine were found to occur at 19 and 22 h, whereas a single peak at 17 h was noticed in the amoebae labelled with [3H]lysine. An almost 2-fold increase of the labelled amino acid incorporation occurred during the peak periods, while the other periods showed a more or less steady state of incorporation, suggesting a basal rate of synthesis at these times. In a detailed study involving the peaks and the basal incorporation period of [3H]leucine, it was shown that the removal of the nucleus or Actinomycin D treatment eliminated the peaks but the base line protein synthesis was not affected. This suggests that for the peak synthetic periods, mRNA is probably transcribed concurrently, followed by immediate translation, whereas long-life mRNA accounts for the basal synthetic activity.

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