18S and 25S ribosomal RNA molecules resistant to a 5'-monophosphate dependent exonuclease are produced by a mechanism independent of TOR

It has been previously shown that Saccharomyces cerevisiae yeast cells produce 18S and 25S ribosomal RNA components that are resistant to exonucleases and require a single phosphate at the 5’- end of the RNA. These molecules are produced during the stationary growth phase when TOR activity decreases. We wanted to further define the relationship between TOR and these resistant RNA molecules. Active suppression of TOR activity by rapamycin results in the production of these molecules. Similarly, a TORC1-deleted mutant Saccharomyces cerevisiae produces resistant 18S and 25S in a steady fashion. Thiouracil labeling of these molecules showed that molecules previously produced during the logarithmic growth phase can be converted to this resistant state. Thiouracil uptake assays also revealed that fewer 18S and 25S genes are produced during the stationary phase. The decapping of these molecules converts them back to an exonuclease-sensitive state. These data indicate that the production of exonuclease resistance of 18S and 25S is independent of TOR activity and is perhaps suppressed when TOR is active. Decapping converts them back to an exonuclease-sensitive state, indicating that at the minimum, there is an additional phosphate at their 5’-end. These molecules likely allow the presence of some ribosomes in the nutritional decline phase to maintain protein production.