Parameters controlling transcriptional activation during early Drosophila development.

We studied transcription during the first 14 mitotic cycles of Drosophila development, by gel electrophoresis of RNA pulse-labeled in vivo. Synthesis of rRNA, tRNAs, 5S RNAs, snRNAs, poly(A)+ RNAs, and histone mRNAs is first detectable during cycle 11 or 12. Histone genes are transcribed during S phases, and reach maximal activation in cycle 12, whereas nonhistone genes are transcribed only in G2 periods, and reach maximal activation during late cycle 14. The high transcriptional activity characteristic of cycle 14 can be precociously induced by extending interphase with cycloheximide as early as, but not before, cycle 10. We conclude that all classes of genes become competent for activation during cycle 10, and that subsequent activation is differentially suppressed by functions associated with nuclear division.