Promoter regions of four Balbiani ring genes in Chironomus tentans exhibit a common salivary gland-specific chromatin organisation, which is independent of the rate of transcriptional initiation.

The Balbiani ring (BR) gene family consists of four closely related genes which show some differences in transcriptional regulation. Here we analyse the chromatin structure of the promoter regions of the four BR genes. In cells in which the genes are inactive, digestion of chromatin with micrococcal nuclease suggests for the BR1 gene that precisely positioned nucleosomes are present from position +200 to -600. In salivary gland cells, a distinctly different organisation of the promoter chromatin leads to the appearance of two sites that are hypersensitive to both DNase I and micrococcal nuclease, located at the approximate positions -50 and -240. Nuclease digestion experiments suggest the presence of a nucleosome in the region between the two hypersensitive sites, and nucleosome disruption at the transcription initiation site. The positions of these hypersensitive sites in all BR genes is unaffected by extensive modulation of the rate of transcriptional initiation, although some alterations in the intensity of the hypersensitive sites were observed. We also show that total nucleosome displacement from the transcribing template is seen only when the RNA polymerase density exceeds one polymerase molecule per 100 bp.