Lessons in maturation

Abstract

Two groups provide insights into quality control during ribosome biogenesis, showing that maturation of the pre-40S ribosomal subunit actually requires association with both the 60S subunit and the translation initiation factor eIF5B. A late step in 40S maturation requires the cleavage of 20S pre-ribosomal RNA (rRNA) to mature 18S rRNA by the endonuclease Nob1. Lebaron et al. show that Nob1-mediated cleavage of the 20S pre-rRNA is regulated by pre-rRNA association of Fun12, the yeast homologue of eIF5B. The 60S ribosomal subunit also associates with pre-40S particles, and the authors propose that this is a trigger for Nob1-mediated cleavage. In the second study, Strunk et al. also find that eIF5B promotes association of the 60S subunit with the pre-40S particle, to form an 80S-like particle. However, these are not functional as they lack mRNA and initiator tRNA. The translation termination factor Rli1 is required to dissociate such non-functional complexes and drive formation of a mature ribosome.