Immature large ribosomal subunits containing the 7S pre-rRNA can engage in translation inSaccharomyces cerevisiae

Abstract

Evolution has provided eukaryotes with mechanisms that impede immature and/or aberrant ribosomes to engage in translation. These mechanisms basically either prevent the nucleo-cytoplasmic export of these particles or, once in the cytoplasm, the release of associated assembly factors, which interfere with the binding of translation initiation factors and/or the ribosomal subunit joining. We have previously shown that aberrant yeast 40S ribosomal subunits containing the 20S pre-rRNA can engage in translation. In this study, we describe that cells harbouring the dob1-1 allele, encoding a mutated version of the exosome-assisting RNA helicase Mtr4, accumulate otherwise nuclear pre-60S ribosomal particles containing the 7S pre-rRNA in the cytoplasm. Polysome fractionation analyses revealed that these particles are competent for translation and do not induce elongation stalls. This phenomenon is rather specific since most mutations in other exosome components or co-factors, impairing the 3' end processing of the mature 5.8S rRNA, accumulate 7S pre-rRNAs in the nucleus. In addition, we confirm that pre-60S ribosomal particles containing either 5.8S + 30 or 5.8S + 5 pre-rRNAs also engage in translation elongation. We propose that 7S pre-rRNA processing is not strictly required for pre-60S r-particle export and that, upon arrival in the cytoplasm, there is no specific mechanism to prevent translation by premature pre-60S r-particles containing 3' extended forms of mature 5.8S rRNA.

Cooling-induced SUMOylation of EXOSC10 down-regulates ribosome biogenesis

“…Consistent with this notion, pre-rRNAs from the large subunit have recently been observed in the cytoplasm and polysomes of yeast (Rodríguez-Galán et al 2015).…”

An Mtr4/ZFC3H1 complex facilitates turnover of unstable nuclear RNAs to prevent their cytoplasmic transport and global translational repression

“…Second, while ZFC3H1 knockdown was shown previously to result in reductions in 47S and 45S pre-rRNAs (Tafforeau et al 2013), our data showed no changes in downstream pre-rRNA and mature rRNA levels following ZFC3H1 knockdown. Third, unprocessed pre-5.8S rRNA in fact assembles into 60S ribosomes (Briggs et al 1998), and the resulting immature 60S particles engage in apparently normal translation (Rodriguez-Galan et al 2015). Indeed, polysome disassembly has not been observed in yeast or mammals under conditions that allow accumulation of aberrant pre-rRNA (Briggs et al 1998;Strezoska et al 2000).…”

The small and large ribosomal subunits depend on each other for stability and accumulation

“…Perhaps pre-60S particles are necessary for complete modification rRNA and/or proteins in the 40S subunit, and under-modification could affect the stability and/or export nuclear pre-40S particles. It is also known that mature 40S subunits containing mutated rRNA are degraded (Rodriguez-Galan et al 2015), but there are no known mutations in the 18S sequence of our strains. We speculate that 40S subunits accumulating as free (non-60S bound) particles due to the deficit of 60S subunits could be vulnerable to cytoplasmic nucleases, because the interface region, which is rich in rRNA loops, is exposed in the free subunits, but protected while it is paired with a 60S subunit.…”