Inhibition of Balbiani ring RNA synthesis at the initiation level.

The nucleoside analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, a selective inhibitor of heterogeneous chromosomal RNA synthesis in salivary gland cells of the midge Chironomus tentans, blocks the initiation of transcription in Balbiani rings 1 and 2. The analogue seems to be without appreciable effect on the elongation of growing RNA chains and allows finished molecules to be detached from the chromosomal sites. The RNA chain growth rate, based on measurements of the time required for completion of synthesis of Balbiani ring 1 and 2 RNA molecules after addition of the inhibitor, was estimated to be around 25 nucleotides per second at 18 degrees.