In the myxomycete Physarum polycephalum, tubulin synthesis is subject to mitotic cycle control. Virtually all tubulin synthesis is limited to a 2-h period immediately preceding mitosis, and the peak of tubulin protein synthesis is accompanied by a parallel increase in the level of tubulin mRNA. The mechanism by which the accumulation of tubulin mRNA is turned on and off is not clear. To probe the relationship between tubulin regulation and cell cycle controls, we have used heat shocks to delay mitosis and have followed the pattern of tubulin synthesis during these delays. Two peaks of tubulin synthesis are observed after a heat shock. One occurs at a time when synthesis would have occurred without a heat shock, and a second peak immediately precedes the eventual delayed mitosis. These results are clearly due to altered cell cycle regulation. No mitotic activity is detected in delayed plasmodia at the time of the control mitosis, and tubulin behavior is shown to be clearly distinct from that of heat shock proteins. We believe that the tubulin family of proteins is subject to regulation by a thermolabile mitotic control mechanism but that once the cell has been committed to a round of tubulin synthesis the "tubulin clock" runs independently of the heat sensitive system. In delayed plasmodia, the second peak of synthesis may be turned on by a repeat of the commitment event.

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