SEPARATE STRUCTURAL ELEMENTS WITHIN INTERNAL TRANSCRIBED SPACER-1 OF SACCHAROMYCES-CEREVISIAE PRECURSOR RIBOSOMAL-RNA DIRECT THE FORMATION OF 17S AND 26S RIBOSOMAL-RNA

Structural features of Internal Transcribed Spacer 1 (ITS1) that direc t its removal from Saccharomyces cerevisiae pre-rRNA during processing were identified by an initial phylogenetic approach followed by in vi vo mutational analysis of specific structural elements. We found that S.cerevisiae ITS1 can functionally be replaced by the corresponding re gions from the yeasts Torulaspora delbrueckii, Kluyveromyces lactis an d Hansenula wingei, indicating that structural elements required in ci s for processing are evolutionarily conserved. Despite large differenc es in size, all ITS1 regions conform to the secondary structure propos ed by Yeh et al. [Biochemistry 29 (1990) 5911-5918], showing five doma ins (I-V; 5'-->3') of which three harbour an evolutionarily highly con served element. Removal of most of domain II, including its highly con served element, did not affect processing. In contrast, highly conserv ed nucleotides directly downstream of processing site A2 in domain III play a major role in production of 17S, but not 26S rRNA. Domain IV a nd V are dispensable for 17S rRNA formation although an alternative, a lbeit inefficient, processing route to mature 17S rRNA may be mediated by a conserved region in domain IV. Each of these two domains is indi vidually sufficient for efficient production of 26S rRNA, suggesting t wo independent processing pathways. We conclude that ITS1 is organized into two functionally and structurally distinct halves.