UC Santa Cruz

Life relies on accurate protein synthesis and the fidelity of mRNA translation is crucial to maintain cellular homeostasis. Subtle errors in mRNAs are often not detected before translation initiation and can result in production of toxic proteins if left unchecked. These mRNAs are regulated by translation surveillance pathways, including mRNAs lacking stop codons (nonstop decay) and mRNAs that block translation elongation (no-go decay). Central to nonstop and no-go decay is the process of ribosome stalling, where trailing ribosomes collide behind the first stalled ribosome, generating a “traffic jam” of ribosomes. Ribosome collisions on an erroneous mRNA trigger decay of both the offending mRNA and the nascent peptide chain to prevent deleterious outcomes, such as ribosome sequestration and production of dominant negative proteins. However, the mechanisms by which decay machinery recognizes aberrant mRNAs are not well understood. Therefore, I performed a genetic screen to identify factors required for nonstop mRNA decay in C. elegans.

Here I report the mutants isolated from the genetic screen. We discovered a novel endonuclease, NONU-1, that is required for both nonstop and no-go mRNA decay. The domain architecture of NONU-1 is conserved throughout metazoans and homologs in S. cerevisiae have a conserved function in nonstop decay. NONU-1 contains an Smr RNase domain that is required for the formation of mRNA cleavage fragments in the vicinity of stalled ribosomes, though a subset of cleavages persist in nonu-1 mutants. Therefore, we propose that another functionally redundant nuclease exists to suppress nonstop and no-go mRNAs.

We also identified the homolog of the ribosome rescue factor Hbs1, K07A12.4. K07A12.4 contains a conserved GTPase domain that is required to repress nonstop mRNAs. Most eukaryotes also express Ski7, a paralog of Hbs1 that is required to facilitate interactions between the SKI helicase complex and the RNA exosome to degrade nonstop mRNAs. Depending on the organism, Ski7 is either an alternative splice isoform of the HBS1 gene or a separate paralogous gene. However, k07a12.4 does not encode a Ski7-like protein and a candidate Ski7 gene has not been identified in C. elegans. New alleles of genes encoding components of the SKI complex, skih-2 and ttc-37, were also isolated from the genetic screen. The collection of mutants will be useful to understand SKI’s role in nonstop and no-go mRNA repression.