Post-replicative nonribosomal transcription units in D. melanogaster embryos.

We have analyzed transcription units from D. melanogaster embryos in which most cells were in either the late S or the G2 phase of a synchronous cell cycle. Much of the chromatin of these embryos was observed in the form of parallel strands which appear to be newly replicated sister chromatids (McKnight and Miller, 1977). Lateral ribonucleoprotein (RNP) fibers were commonly observed attached to chromatin strands and are believed to be nascent transcripts. Very similar lateral fiber arrays were found on sister chromatid strands. Such paired, post-replicative RNP fiber arrays offer a unique opportunity to compare the transcription patterns on two copies of the same transcription unit. Analyses of paired transcription units yield the following conclusions. Transcription initiation and termination sites are the same for both copies of each transcription unit. Each transcription unit has a characteristic and constant RNP fiber frequency, and the fiber frequency is identical for both copies of the transcription unit. Active transcription units can have internal, fiber-free gaps which may result from interruptions in initiation of transcription. Transcription units differing in polarity and fiber frequency can occur in immediate juxtaposition. The relevance of these findings to eucaryotic transcriptional control mechanisms are discussed.