Co-transcriptional architecture in a Y loop in Drosophila melanogaster.

The Y loops of Drosophila spermatocytes are formed by the expression of huge individual transcription units on the Y chromosome and their large size provides a unique system for the investigation of the organisation of transcription in intact nuclei. By labelling ribonucleo-protein (RNP) components, the loop chromatin and nascent transcripts in Y loop C, we reveal a highly structured organisation of RNP domains associated with nascent transcripts. We distinguish two types of RNP domain, a proximal domain that runs alongside the chromatin of loop C and a distal RNP domain that wraps around the proximal domain and the loop chromatin. The proximal domain is marked by the Pasilla protein, and separate distal subdomains are marked by the S5 antigen and Boule. We discuss the implications of this highly structured co-transcriptional architecture for the organisation of the process of transcription.