DNA compaction during intense transcription measured by electron microscope tomography.

Thick sections (0.5-1.0 micron) containing pilocarpine-hyperstimulated Balbiani ring (BR) genes of Chironomus tentans salivary glands were reconstructed by electron microscope tomography (EMT). Six transcription loops were analyzed at low resolution permitting a measurement of transcription unit length in situ. Comparison of the average transcription unit length with the BR gene coding region DNA length yields an estimate of the DNA compaction ratio in situ. The estimated DNA compaction ratio (approximately 3-fold) is consistent with a significant degree of nucleosome unfolding in the hyperstimulated BR genes.