Complex endonucleolytic cleavage pattern during early events in the processing of pre-rRNA in the lower eukaryote, Tetrahymena thermophila.

We have analysed nuclear RNA from T. thermophila by RNA transfer hybridization using cloned rDNA fragments. A very high number of in vivo intermediates and by-products of rRNA processing were identified. These include putative intermediates of the splicing process and alternative products resulting from temporal variability in various endonucleolytic cleavages. In addition, four small RNA species including only transcribed spacer sequences were detected. These are (1) the IVS RNA (approximately 400 bases), the by-product of the splicing process, (2) a fragment from the internal transcribed spacer (approximately 360 bases), possibly resulting from 3'-end processing of pre-17S rRNA, (3) a fragment comprising most or all of the external transcribed spacer (approximately 600 bases) obviously representing the major by-product of 5'-end processing, and, in addition, (4) a small fragment from the initiation region (approximately 230 bases) which might be a product of premature transcription termination.