From knobs to a central pseudoknot : understanding 40S ribosomal subunit biogenesis through Bud23
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Ribosomes are universally conserved macromolecular machines that translate cellular genetic information into proteins. All ribosomes are com- posed of two ribonucleoprotein subunits. In eukaryotes these are called 40S (small) and 60S (large) subunits. Biogenesis of both subunits begins from a common precursor ribosomal RNA (rRNA) transcript in the nucleolus. The 18S rRNA of the small subunit is encoded in the 5ʹ end of the precursor transcript. U3 snoRNA and about 70 accessory factors associate with the 50 end of the pre-rRNA, to form the SSU processome or 90S pre-ribosome, which can be observed as terminal knobs in electron micrographs. After the initial processing and folding, the pre-rRNA is cleaved at site A2 to release the pre--40S. This event is dependent on the formation of the central pseudoknot, a structure that maintains the integrity of 40S architecture. Bud23 is the methyltransferase responsible for modification of the base G1575 in the P-site of the small subunit. Work presented here demonstrates that the in vivo stability, and thus function, of Bud23 is dependent on the presence of Trm112, a novel ribosome biogenesis factor identified in this work. Analysis of rRNA processing and strong negative genetic interactions with RNaseMRP mutants, provide strong evidence for that BUD23 is required for A2 cleavage. Extragenic suppressors of bud23 [delta] were identified in UTP14, UTP2, IMP4 and ECM16, coding for SSU processome components. Bud23 and the RNA helicase Ecm16 interact physically as well as genetically. Most fascinatingly, using ecm16 enzymatic mutants, this work provides compelling evidence that Ecm16 facilitates removal of U3 snoRNA from pre-rRNA, a prerequisite for central pseudoknot formation and 90S to pre--40S transition. These findings suggest a model in which binding of Bud23 monitors the status of 40S assembly, triggering Ecm16 activity to promote release of the pre--40S from 90S only after the critical folding of the small subunit rRNA.