Rob Aerts, Tony Durston and Wouter Moolenaar, Hubrecht Laboratory, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands

We measured cytoplasmic pH in Dictyostelium discoideum under various conditions, using the digitonin null-point method (Rink, T. J., Tsien, R. Y., Pozzan, T. (1982) J. Cell Biol., 95,189. In synchronised cell populations the cytoplasmic pH clearly fluctuates during the cell cycle. These fluctuations coincide with fluctuations in the rates of biosynthetic processes; the rates of DNA synthesis and protein synthesis show a sharp optimum in the cytoplasmic pH range 7-40 to 7-45.

The cytoplasmic pH of Dictyostelium discoideum can be changed by certain weak acids and bases. The effect of ammonia is concentration dependent; a low concentration is able to increase cytoplasmic pH, high concentrations have the opposite effect. Artificial manipulation of cytoplasmic pH influences the rate of DNA and protein synthesis and shows the same correlation between these processes and cytoplasmic...

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