Visualization of active 75 S RNA genes in the Balbiani rings of Chironomus tentans.

The active 75 S RNA genes in the Balbiani rings of Chironomus tentans were investigated by two complementing electron microscopy procedures: spreading of isolated chromosomes according to Miller and serial sectioning of Balbiani rings. The Miller spreads show that the transcription products, the ribonucleoprotein (RNP) fibers, increase gradually in length along the 75 S RNA genes. The morphology of the active genes in situ suggests the following sequence of events: during transcription a 20 nm RNP fiber is formed which is oriented roughly perpendicular to the chromosomal axis. When an RNP fiber corresponding to one quarter of the gene has been generated, packaging of the fiber into a dense globular structure begins at the free end of the RNP fiber. The processes of transcription and packaging go on in parallel in such a way that the length of the 20 nm fiber is kept constant, while the globular part is steadily increasing in size. When transcription is completed, a globular product is released, 50 nm in diameter. A thin (5 nm) chromosomal axis as well as a low DNA compaction (3.6) imply that the DNA in the chromosome fiber of an active 75 S RNA gene is more extended than the DNA in a nucleofilament. On the basis of our electron microscopy data and other information available on the Balbiani ring genes, we discuss the activation of the 75 S RNA genes in terms of a two-step process.