Actinomycin D-sensitive periods in the differentiation of Drosophila neurons and muscle cells in vitro.

Observations were made of neuroblasts differentiating into neurons, and myoblasts differentiating into myocytes in cultures of embryonic Drosophila cells. Axons greater than 50 mum long appeared in vitro between 7.5 and 16.5 h, and pulsating myocytes appeared between 12.5 and 23.5 h. Actinomycin D treatment prevented neuroblasts and myoblasts from proceeding in differentiation. Neurons became resistant to actinomycin D when they reached the stage of axon initiation, and axon elongation was not actinomycin D-sensitive. Myocytes were sensitive to actinomycin D until they attained the ability to pulsate, and ongoing pulsations were not halted by the drug. Autoradiographs and controls indicated that actinomycin D prevented uridine incorporation by about 90% but prevented leucine and thymidine incorporation by 6% or less in these cells. The result favour the interpretation that transcription is necessary in neuron differentation up to the point of axon initiation and in myocytes up to the point where pulsations can begin. Since ribosomal RNA synthesis is unnecessary for these differentiations, necessary RNA synthesis would be messenger or transfer RNA, or RNA of unknown identity.