Stimulation of cytoplasmic actin gene transcription and translation in cultured Drosophila cells by ecdysterone.

When cultured Drosophila line S3 cells are incubated in the presence of the steroid hormone, ecdysterone, they flatten, elongate, and become motile. We show here that accompanying this morphological transformation there is a 5-fold increase in the rate of actin synthesis and a 2-fold increase in actin content. These increases, in turn, are primarily based on a 9-fold increase in the level of mRNA transcribed from the cytoplasmic actin A3 gene. We also find that during hormone treatment the cells go into proliferative arrest, accumulating in the G2 phase of the cell cycle. During this period the level of histone mRNA in the cells decreases significantly.