Packing of a specific gene into higher order structures following repression of RNA synthesis.

Transcription of the Balbiani ring (BR) genes of the dipteran Chironomus tentans was inhibited by teh nucleoside analogue DRB (5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole). The BR genes were emptied of RNA polymerases and the subsequent packing of the genes was monitored by transmission electron microscopy. The thin chromatin axis of the transcriptionally active genes condensed into a thick (20-25 nm) chromatin fiber, which was recorded as a linear structure, an open loop or a supercoiled loop. The compacted genes were finally packed into dense clumps of chromatin. It was proposed that upon repression of RNA synthesis the BR gene template attains the following consecutive stages with increasing compaction: transcription loop----linear thick fiber----open thick fiber loop----supercoiled thick fiber loop----dense chromatin. Within the chromatin blocks structures that resembled the supercoiled loops were discerned, suggesting that the final packing of the template might be accomplished by a close alignment of supercoiled loops.