The transcription termination site of the ribosomal RNA operon in yeast

The site at which transcription of the ribosomal RNA operon in yeast is terminated was precisely localized. First, the exact position of the 3' end of the 26S rRNA gene was mapped on the rDNA on the basis of RNA- and DNA sequence data. Next, the 3' terminus of the primary transcript, 37S precursor rRNA, was established by hybridization experiments and sequence analysis. 37S pre-rRNA appears to be just 7 nucleotides longer at its 3' end than 26S rRNA. The non-coding strand around the termination site is extremely T-rich: 15 out of 18 nucleotides are T-residues. An extensive dyad symmetry is present in the sequence downstream from the termination site; a possible role of this structure in the regulation of transcription termination is discussed. The 3'-terminal 110 nucleotides of yeast 26S rRNA have approx. 50% and 60% homology with the corresponding regions of E. coli 23S rRNA and Xenopus laevis 28S rRNA, respectively.