68 research outputs found
Thermophile 90S Pre-ribosome Structures Reveal the Reverse Order of Co-transcriptional 18S rRNA Subdomain Integration
The âbirthâ of the eukaryotic ribosome is preceded by RNA folding and processing reactions that depend on assembly factors and snoRNAs. The 90S (SSU-processome) is the earliest pre-ribosome structurally analyzed, which was suggested to assemble stepwise along the growing pre-rRNA from 5â>3â, but this directionality may not be accurate. Here, by analyzing the structure of series of novel 90S assembly intermediates isolated from Chaetomium thermophilum, we discover a reverse order of 18S rRNA subdomain incorporation. This revealed that large parts of the 18S rRNA 3â and central domains assemble first into the 90S, before the 5â domain is stably integrated. This final incorporation depends on a physical contact between a heterotrimer Enp2-Bfr2-Lcp1 recruited to the flexible 5â domain and Kre33, which reconstitutes the Kre33-Enp-Brf2-Lcp5 module on the compacted 90S pre-ribosome. Keeping the 5â domain temporarily segregated from the 90S scaffold could provide an extra time to complete the multifaceted 5â domain folding, which depends on a distinct set of snoRNAs and processing factors
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