Structure of the Tetrahymena pyriformis rRNA gene. Nucleotide sequence of the transcription initiation region.

Portions of rRNA gene repeat from Tetrahymena pyriformis were cloned in pBR322, and a subclone containing the transcription initiation site was isolated. The promoter region was identified by S1 nuclease mapping of a portion of the HindIIIB fragment of the rRNA gene, using the 35 S rRNA primary transcript as a probe. The nucleotide sequence of the initiation site and the 5'-flanking region reveals three sets of highly conserved repeat sequences. A model of the promoter region is presented in which the rDNA is wrapped around a series of histone octamers. In this model the families of repeat sequences are brought in close proximity and suggest several possible mechanisms for the regulation of rRNA synthesis in T. pyriformis.