18S and 25S Exonuclease Resistant Ribosomal RNA Molecules Are Produced by 5'-End Modification During TOR Inhibition

Saccharomyces cerevisiae yeast cells have been shown to produce 18S and 25S ribosomal RNA molecules that are resistant to degradation by exonucleases, which require a 5' monophosphate for activity. These resistant RNA species accumulate during the diauxic shift, a phase marked by reduced TOR signaling. To further investigate the link between TOR activity and the accumulation of resistant rRNA, we examined the effects of pharmacological TOR inhibition. Treatment with rapamycin, an active TOR suppressor, led to increased levels of resistant 18S and 25S RNA. Importantly, this accumulation was also observed in cells with constitutively active RNA polymerase I (CARA), indicating that the resistant RNA species arise independently of RNA Pol I transcriptional regulation. Similarly, a TOR1-deleted mutant of Saccharomyces cerevisiae produces resistant 18S and 25S rRNA species in a sustained manner. Thiouracil labeling revealed that rRNA molecules generated during the logarithmic growth phase can be converted into the resistant form, suggesting a posttranscriptional modification process. Furthermore, thiouracil uptake assays demonstrated that overall rRNA synthesis decreases during the diauxic phase. Notably, decapping of the resistant rRNAs restored their sensitivity to exonucleases, indicating that the resistance is conferred by 5' end modifications, likely involving the addition of one or more phosphate groups.