TETRAHYMENA RIBOZYME DISRUPTS RIBOSOMAL-RNA PROCESSING IN YEAST

The intervening sequence (IVS) of Tetrahymena thermophila nucleolar DNA interrupts a highly conserved sequence in the RNA core structure of the large ribosomal subunit. This location in nuclear DNA is unusual as most group I introns are in mitochondrial and chloroplast DNA. To examine the effect of a ribozyme insertion in another nuclear genome, the Tetrahymena NS was introduced into the analogous position in a cloned Schizosaccharomyces pombe ribosomal gene, and the mutant rDNA was expressed in vivo. RNA analyses indicated that mature 5.8 S rRNA was not formed from the mutant gene transcript and the amount of 27 S nRNA was significantly reduced. In contrast, hybridization analyses indicated that RNA splicing continued, and normal forms of free ribozyme were present. The results show that the TVS sequence can interfere with rRNA processing and suggest that the unusual amplification of a single rDNA repeat may have forced Tetrahymena to accommodate its ribozyme.