Morphology of transcription units in Drosophila melanogaster.

We have used an electron microscopic analysis to define and to characterize active transcription units of Drosophila melanogaster. The lengths and spacings of nascent ribonuclear protein (RNP) fibers were determined on embryonic chromatin that was spread using techniques introduced by Miller and Beatty (1969). The data are consistent with the occurrence of specific sites of transcription initiation and termination. We apply the term transcription unit (TU) to a chromatin region bounded by these control sites. Two classes of TUs are active in Drosophila melanogaster embryonic cells--those synthesizing ribosomal RNA and those synthesizing non-ribosomal RNA. The classes can usually be distinguished on the basis of TU size, chromatin morphology and inferred DNA packing ratio, frequency of RNP fibers (number of fibers per mum of chromatin), and the solitary vs. tandem repeat occurrence of fiber arrays. The results indicate that non-ribosomal transcription units have lengths in accord with the expectation that DNA of each chromomere is transcribed as a unit. Some nascent fiber arrays in D. melanogaster have more complex patterns of RNP fiber lengths. We suggest that these are a consequence of cleavage of RNP fibers at specific sites during transcription. These sites of transcriptional control and the amounts of DNA between them provide a basis for further relating units of transcription to units of gene function.