AbstractTwo isogenic acyclic strains ofParamecium multimicronucleatum, syngen 2, expressing mating types III and IV, and a cycler strain, which displays circadian alternation of the two mating types, were treated with proteolytic enzymes, or the inhibitors actinomycin D or puromycin at various stages of the mating reactive period induced by food depletion. Reactivity of acyclic animals was sensitive to proteolytic enzymes, and to both inhibitors before or after the onset of reactivity, suggesting that reactivity is attained and maintained via continuous RNA and protein synthesis, and that protein integrity is essential to the activity of the type‐specific substances. A new cell cycle was not necessary for recovery from a block in reactivity.Type III animals were consistently more sensitive to inhibition by puromyin, and required longer recovery periods after all treatments except actinomycin D given after the development of reactivity. Possible explanations of these observations are considered.The daily type changes seen in cycler animals seemed to occur by replacement of one mating type substance with the other. Type‐switching was neither prevented nor displaced by exposure to actinomycin D or puromycin during a period before or including a type change, even though reactivity was reduced by more than 95%, suggesting that type‐switching is not directly associated with transcription or translation of the type‐specifi
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