Common sequence elements are important for transcription and replication of the extrachromosomal rRNA-encoding genes...

Three types of conserved sequence elements have previously been identified in the generally non-conserved, 5' non-transcribed spacer region of the rRNA-encoding gene (rDNA) of Tetrahymena species. In vivo and in vitro experiments have identified the start point (tsp) for rDNA transcription. Comparative sequence analysis suggested that the type-I sequence element was likely to function as the promoter for rDNA transcription. We have used in vitro transcription assays to demonstrate that sequences in the 3' half of the type-I repeat located proximal to the tsp are essential for the accurate initiation of rRNA synthesis. These sequences define the 5' boundary of the core or minimal promoter element and include the sequences which function in rDNA replication and maintenance in the homologous regions in two upstream type-I sequence elements. These sequences may bind factors important for both the transcription and replication of rDNA.