Rapid reformation of the thick chromosome fiber upon completion of RNA synthesis at the Balbiani ring genes in Chironomus tentans

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Abstract
We have studied the ultrastructure of the Balbiani ring genes in Chironomus tentans during treatment with the RNA synthesis inhibitor DRB (5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole). This nucleoside analogue blocks transcription at or near the initiation site but does not interfere with the elongation and termination processes. In the ordinary active state the Balbiani ring genes display a 5 nm chromosome fiber, carrying densely distributed, growing ribonucleoprotein particles (Andersson et al., 1980). When the transcriptional activity declines, a 10 nm fiber can be observed between sparsely distributed RNA polymerases. Furthermore, after passage of the last RNA polymerase the 10 nm fiber can be seen as well as its gradual packing into a 25 nm thick fiber. Thus, the active chromosome fiber is rapidly packed into higher order structures when the fiber is not directly involved in transcription. The formation of the thick fiber does not require that the gene along its entire length is devoid of active RNA polymerases. The thick fiber can again be mobilized for transcription, since in reversion experiments the BR genes appear as ordinary active genes with an extended nucleofilament and densely packed nascent transcription products. The dynamic behaviour of the chromosome fiber during transcription is discussed as well as the packing and unpacking of a gene into higher order structures.