Correlation between tubulin mRNA stability and poly(A) length over the cell cycle of Physarum polycephalum

During the cell cycle of the Physarum polycephalum plasmodium, levels of α-tubulin mRNA rise exponentially in G 2 phase, reach a peak at metaphase 40-fold above basal levels, and then fall exponentially to basal levels after mitosis. We show that post-mitotic α-tubulin mRNA carries poly(A) tracts of less than 30 residues. By contrast, when levels of α-tubulin mRNA rise during G 2 phase, the mRNA has a poly(A) tract of approximately 80 bases. The length of the poly(A) tract of any mRNA encoding actin is relatively constant at fewer than 30 bases through the cycle. We have estimated the apparent rate of synthesis of α-tubulin mRNA at different stages of the cell cycle by short-term labeling in vivo. Transcription of α-tubulin mRNA continues even after mitosis, though the rate may be diminished relative to that in late G 2 phase. So, the post-mitotic molecular half-life of α-tubulin mRNA must be less than the 19 minute half-life by which the levels of this species fall. The fact that the apparent rate of α-tubulin mRNA synthesis is not vastly greater in early G 2 phase than in post-mitotic plasmodia is consistent with an S-phase destabilization of α-tubulin mRNA molecules. Thus, the poly(A) tail is shorter when the α-tubulin mRNA is less stable.