Wang, C.; Sunder, S.; Johnson, A.

25S nonfunctional RNA decay (NRD) eliminates 60S ribosomal subunits carrying inactivating mutations in the RNA. However, how cells identify defective subunits has not been described. We recently showed that the zinc-finger protein Reh1 is the last assembly factor to be released from a nascent 60S subunit. We now show that in yeast Reh1 is required for the degradation of 25S NRD substrates. 25S rRNAs carrying mutations in the catalytic center, A2820G or U2954A (A2451 and U2585, respectively in E coli numbering), are unstable in wildtype cells but are fully stabilized when REH1 is deleted. However, not all 25S rRNA mutations are recognized by Reh1. Ribosomes with a truncated L1 stalk engage in translation but cannot support viability. These ribosomes display a half-life indistinguishable from wild-type rRNA, suggesting that yeast does not have a robust surveillance system for such mutant ribosomes. Deletion of REH1 also has no impact on the levels of defective 18S rRNA. These results indicate that Reh1 and 25S NRD are specific for mutations in or near the catalytic center of the ribosome.