Presence of histone H1 on an active Balbiani ring gene.

We have investigated whether histone H1 is present on active Balbiani ring genes in the salivary glands of Chironomus tentans using immunoelectron microscopy. The genes were studied in two activity states: maximally activated genes with a fully extended template and repressed genes in a 30 nm fiber conformation. Histone H1 was recorded on the gene in both conformations; the immunosignal was considerably stronger in the transcriptionally active state, probably reflecting the increased accessibility of histone H1 to the antibody in unfolded versus compacted chromatin. We conclude that during transcription the DNA template is extended and the nucleosomes are disrupted at the RNA polymerases, but histone H1, and most likely also the core histones, remains bound to the template.