Posttranscriptional and posttranslational modification of macromolecules is known to fine-tune their functions. Trm112 is unique, acting as an activator of both tRNA and protein methyltransferases. Here we report that in Saccharomyces cerevisiae, Trm112 is required for efficient ribosome synthesis and progression through mitosis. Trm112 copurifies with pre-rRNAs and with multiple ribosome synthesis trans-acting factors, including the 18S rRNA methyltransferase Bud23. Consistent with the known mechanisms of activation of methyltransferases by Trm112, we found that Trm112 interacts directly with Bud23 in vitro and that it is required for its stability in vivo. Consequently, trm112⌬ cells are deficient for Bud23-mediated 18S rRNA methylation at position G1575 and for small ribosome subunit formation. Bud23 failure to bind nascent preribosomes activates a nucleolar surveillance pathway involving the TRAMP complexes, leading to preribosome degradation. Trm112 is thus active in rRNA, tRNA, and translation factor modification, ideally placing it at the interface between ribosome synthesis and function.A ctively growing budding yeast cells produce an average of 33 ribosomes per second, which is considerable (61). Besides the synthesis of its constituents, i.e., 79 ribosomal proteins and 4 rRNAs, ribogenesis involves a large number of so-called transacting factors, including proteins and small nucleolar RNAs (13, 56). These interact only transiently with preribosomes and are required for pre-rRNA processing (i.e., cleavages), pre-rRNA modification, and preribosome assembly and transport.Ribosome synthesis is initiated in the nucleolus, a highly dynamic specialized subcompartment of the nucleus organized around clusters of actively transcribed rRNA genes (60). There, RNA polymerase I produces a 35S/47S (in yeasts and humans, respectively) primary transcript which is processed through a complex succession of endo-and exoribonucleolytic cleavages into three of the four mature rRNAs, the 18S, 5.8S, and 25S/28S rRNAs. The fourth rRNA, 5S, is independently transcribed by RNA polymerase III. Ribosome maturation starts cotranscriptionally in the nucleolus, progresses in the nucleoplasm until preribosomes reach the nuclear pore complex, and is finalized in the cytoplasm. Cytoplasmic maturation steps consisting of pre-rRNA processing and structural reorganization result in the assembly of prominent ribosomal structures, such as the "beak" of the small subunit (SSU) and the "stalk" of the large subunit, and are a prerequisite to the acquisition of functionality (43).It is not clear how the various facets of ribosome synthesis are integrated with ribosome function. However, there is growing evidence that such coordination exists. For the small subunit, recent cryoelectron microscopy (cryo-EM) maps of late pre-40S subunits indicate that the binding of trans-acting factors literally mask functional sites, preventing premature translation initiation (57). For the large subunit, trans-acting factors that resemble ribosomal proteins (suc...

Terminal balls detected at the 59-end of nascent ribosomal transcripts act as pre-rRNA processing complexes and are detected in all eukaryotes examined, resulting in illustrious Christmas tree images. Terminal balls (also known as SSU-processomes) compaction reflects the various stages of cotranscriptional ribosome assembly. Here, we have followed SSU-processome compaction in vivo by use of a chromatin immunoprecipitation (Ch-IP) approach and shown, in agreement with electron microscopy analysis of Christmas trees, that it progressively condenses to come in close proximity to the 59-end of the 25S rRNA gene. The SSU-processome is comprised of independent autonomous building blocks that are loaded onto nascent pre-rRNAs and assemble into catalytically active pre-rRNA processing complexes in a stepwise and highly hierarchical process. Failure to assemble SSU-processome subcomplexes with proper kinetics triggers a nucleolar surveillance pathway that targets misassembled pre-rRNAs otherwise destined to mature into small subunit 18S rRNA for polyadenylation, preferentially by TRAMP5, and degradation by the 39 to 59 exoribonucleolytic activity of the Exosome. Trf5 colocalized with nascent pre-rRNPs, indicating that this nucleolar surveillance initiates cotranscriptionally.

Ribosome synthesis entails the formation of mature rRNAs from long precursor molecules, following a complex pre-rRNA processing pathway. Why the generation of mature rRNA ends is so complicated is unclear. Nor is it understood how pre-rRNA processing is coordinated at distant sites on pre-rRNA molecules. Here we characterized, in budding yeast and human cells, the evolutionarily conserved protein Las1. We found that, in both species, Las1 is required to process ITS2, which separates the 5.8S and 25S/28S rRNAs. In yeast, Las1 is required for pre-rRNA processing at both ends of ITS2. It is required for Rrp6-dependent formation of the 5.8S rRNA 3= end and for Rat1-dependent formation of the 25S rRNA 5= end. We further show that the Rat1-Rai1 5=-3= exoribonuclease (exoRNase) complex functionally connects processing at both ends of the 5.8S rRNA. We suggest that prerRNA processing is coordinated at both ends of 5.8S rRNA and both ends of ITS2, which are brought together by pre-rRNA folding, by an RNA processing complex. Consistently, we note the conspicuous presence of ϳ7-or 8-nucleotide extensions on both ends of 5.8S rRNA precursors and at the 5= end of pre-25S RNAs suggestive of a protected spacer fragment of similar length. Ribosomes are essential to all life forms. Ribogenesis is a major metabolic activity requiring the coordinated expression of the core RNA and protein components of the small and large subunits and also of a myriad of trans-acting protein and RNA factors, their maturation, packaging, and transport (reviewed in references 21, 29, and 42). Despite this great complexity, ribogenesis is an extremely robust process. Quality control and fail-safe mechanisms are available at all steps along the assembly pathway to ensure that a sufficient amount of functional ribosomes is provided at all times (reviewed in references 24 and 30).In eukaryotes, ribogenesis is initiated in the nucleolus. There, at the interface between the cortical side of fibrillar centers (FCs) and the surrounding dense fibrillar components (DFCs), RNA polymerase I is recruited and threaded along the ribosomal DNA (rDNA), producing pre-rRNA transcripts that extend into the DFC, where pre-rRNAs undergo cotranscriptional modification. In fast-growing budding yeast cells, up to 50 to 70% of pre-rRNA molecules are also subjected to cotranscriptional cleavage (3,23,28,36). Cotranscriptional cleavage occurs in internal transcribed spacer 1 (ITS1), separating the precursors destined to mature into the small and large ribosome subunit rRNAs, i.e., the 18S and 5.8S-25S rRNAs, respectively (28,36,47). This strategy, involving the cosynthesis of three of the four mature rRNAs from a single long transcript, contributes to coordinating the synthesis of the various components of the translational machinery. This strategy has been extensively conserved throughout evolution and predates the eukaryotes, since the Bacteria and Archaea also synthesize their rRNAs from polycistronic precursors (reviewed in reference 31).The synthesis of mature rRNAs relie...