Temperature-dependent actinomycin D effect on RNA synthesis during synchronous development in Neurospora crassa

Actinomycin D at a concentration of 5 g/ml of medium inhibited DNA-dependent RNA synthesis by 92% at 35 C, 42% at 30 C, and 28% at 25 C in Neurospora crassa. This concentration also inhibited the development of conidiophores and conidia at 35 C, but not at 30 or 25 C. Mycelia which were induced to synchronous development formed conidiophores in 2.5 hr and conidia in 4.5 hr at 35 C in the absence of drug additives. Addition of actinomycin D to synchronously developing mycelia at zero time and at 0.5-hr intervals thereafter at 35 C indicated that RNA synthesis required for conidiophores occurred before 0.5 hr and for conidia before 2 hr. Addition of cycloheximide at the same times to another synchronous mycelial series at 35 C indicated that protein synthesis required for conidiophores occurred before 2 hr and for conidia before 3.5–4 hr.This work was supported in part by U.S. Public Health Service Training Grant 1 TO1 GM 01968 01.