5′-End formation of yeast 5.8SL rRNA is an endonucleolytic event

Like most eukaryotes, Saccharomyces cerevisiae cells contain a minor 5.8SL rRNA that, relative to the major 5.8SS species, carries several extra nucleotides at the 5′-end. The two species are produced by alternative pathways that differ in the events removing the 3′-terminal region of Internal Transcribed Spacer 1 from the 27SA2 pre-rRNA. Whereas the pathway leading to 5.8SS rRNA is well established, that producing the 5′-end of 5.8SL (called B1L) is poorly understood. Northern analysis of two different mutants of S. cerevisiae that overproduce 5.8SL rRNA revealed the presence of a fragment corresponding to the 3′-terminal region of Internal Transcribed Spacer 1 (ITS1) directly upstream from site B1L. Immunoprecipitation experiments showed this fragment to be associated with the trans-acting factor Rrp5p required for processing at the early sites A0–A3. Together these data clearly support that the 5′-end of 5.8SL rRNA is an endonucleolytic event. In vivo mutational analysis demonstrated the lack of any cis-acting sequence elements directing this cleavage within ITS1.