The transcribed template and the transcription loop in Balbiani rings.

The active Balbiani ring (BR) genes in the giant puffs in Chironomus salivary glands can be visualized in three dimensions within the cell nucleus. The chromatin template exhibits a dynamic structure: at the RNA polymerases the template is fully extended, while in between polymerases it can be packed into a nucleosome, a nucleosome filament, or even into a thick chromatin fiber, if the spacing between consecutive polymerases is large enough. Even in its extended state, the template harbours histone H1, and most likely also the core histones, indicating that the nucleosomes unfold and refold during transcription. Apart from the transcribed template, a transcription loop contains a thin, extended, apparently flexible and nucleosome-free fiber upstream of the gene, and a loosely coiled nucleosome filament downstream of the gene.