# 18S and 25S Exonuclease Resistant Ribosomal RNA Molecules Are Produced by 5′‐End Modification During TOR Inhibition

## TL;DR

Yeast cells produce ribosomal RNA molecules resistant to enzyme digestion when TOR signaling is inhibited, likely due to 5′-end modifications that help them survive under stress.

## Contribution

The study identifies 5′-end modifications as the cause of exonuclease resistance in rRNA during TOR inhibition in yeast.

## Key findings

- Resistant 18S and 25S rRNA accumulate during TOR inhibition and the diauxic shift.
- Resistance is conferred by 5′-end modifications, as decapping restores exonuclease sensitivity.
- Resistant rRNA can form posttranscriptionally, even in cells with active RNA polymerase I.

## Abstract

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.

RNA modifications are crucial in the regulation of gene expression and the stability of RNA molecules.In this paper, we describe previously discovered rRNA molecules that are produced by the yeast Saccharomyces cerevisiae when the cells are exposed to a nutritionally deprived environment.Our main goal was to find what causes these rRNA molecules to become resistant to enzyme digestion. We found the presence of extra phosphates at the beginning of both 18S and 25S species. This resistant capacity, observed when the cells are under stress conditions, might allow the organisms to make proteins needed to survive.

RNA modifications are crucial in the regulation of gene expression and the stability of RNA molecules.

In this paper, we describe previously discovered rRNA molecules that are produced by the yeast Saccharomyces cerevisiae when the cells are exposed to a nutritionally deprived environment.

Our main goal was to find what causes these rRNA molecules to become resistant to enzyme digestion. We found the presence of extra phosphates at the beginning of both 18S and 25S species. This resistant capacity, observed when the cells are under stress conditions, might allow the organisms to make proteins needed to survive.

Saccharomyces cerevisiae produces exonuclease‐resistant rRNA molecules upon TOR inhibition. This phenomenon occurs when cells enter the diauxic phase, upon addition of rapamycin to the culture, or when TOR activity is reduced through deletion of the TOR1 gene. A 5′‐end modification is implicated in this process, as mid‐log phase cells acquire this resistance, as confirmed by thiouracil‐chase experiments.