Effect of actinomycin D on cultural growth phases and on the pattern of total RNA synthesis in Tetrahymena

Actinomycin D added at inoculation, alters the cultural growth pattern of Tetrahymena pyriformis as follows: (a) duration of lag phase is prolonged, (b) generation time increased, (c) time in log phase increased, (d) time in decelerating growth phase decreased, (e) number of cells entering stationary phase decreased. Addition of the drug after 4 hr of growth does not induce a new lag phase in the populations. Measurements of total RNA indicate that, as expected, RNA is produced at a slower rate in the treated cultures compared to the controls. Also less total RNA is synthesized by stationary phase, when RNA synthesis stops. The onset of RNA synthesis occurs without any apparent lag. During log phase in both control and treated cultures, doubling of RNA does not keep up with doubling of cell number, thus after the first generation of log growth RNA/cell falls.