Nearly 20% of yeast genes are required for viability, hindering genetic analysis with knockouts. We created promoter-shutoff strains for over two-thirds of all essential yeast genes and subjected them to morphological analysis, size profiling, drug sensitivity screening, and microarray expression profiling. We then used this compendium of data to ask which phenotypic features characterized different functional classes and used these to infer potential functions for uncharacterized genes. We identified genes involved in ribosome biogenesis (HAS1, URB1, and URB2), protein secretion (SEC39), mitochondrial import (MIM1), and tRNA charging (GSN1). In addition, apparent negative feedback transcriptional regulation of both ribosome biogenesis and the proteasome was observed. We furthermore show that these strains are compatible with automated genetic analysis. This study underscores the importance of analyzing mutant phenotypes and provides a resource to complement the yeast knockout collection.

The biological role of many nonessential tRNA modifications outside of the anticodon remains elusive despite their evolutionary conservation. We show here that m7G46 methyltransferase Trm8p/Trm82p acts as a hub of synthetic interactions with several tRNA modification enzymes, resulting in temperature-sensitive growth. Analysis of three double mutants indicates reduced levels of tRNA(Val(AAC)), consistent with a role of the corresponding modifications in maintenance of tRNA levels. Detailed examination of a trm8-delta trm4-delta double mutant demonstrates rapid degradation of preexisting tRNA(Val(AAC)) accompanied by its de-aminoacylation. Multiple copies of tRNA(Val(AAC)) suppress the trm8-delta trm4-delta growth defect, directly implicating this tRNA in the phenotype. These results define a rapid tRNA degradation (RTD) pathway that is independent of the TRF4/RRP6-dependent nuclear surveillance pathway. The degradation of an endogenous tRNA species at a rate typical of mRNA decay demonstrates a critical role of nonessential modifications for tRNA stability and cell survival.

More than 170 proteins are necessary for assembly of ribosomes in eukaryotes. However, cofactors that function with each of these proteins, substrates on which they act, and the precise functions of assembly factors-e.g., recruiting other molecules into preribosomes or triggering structural rearrangements of pre-rRNPs-remain mostly unknown. Here we investigated the recruitment of two ribosomal proteins and 5S ribosomal RNA (rRNA) into nascent ribosomes. We identified a ribonucleoprotein neighborhood in preribosomes that contains two yeast ribosome assembly factors, Rpf2 and Rrs1, two ribosomal proteins, rpL5 and rpL11, and 5S rRNA. Interactions between each of these four proteins have been confirmed by binding assays in vitro. These molecules assemble into 90S preribosomal particles containing 35S rRNA precursor (pre-rRNA). Rpf2 and Rrs1 are required for recruiting rpL5, rpL11, and 5S rRNA into preribosomes. In the absence of association of these molecules with pre-rRNPs, processing of 27SB pre-rRNA is blocked. Consequently, the abortive 66S pre-rRNPs are prematurely released from the nucleolus to the nucleoplasm, and cannot be exported to the cytoplasm. In eukaryotes, 79 ribosomal proteins associate with ribosomal RNA (rRNA) to produce 40S and 60S ribosomal subunits (Woolford and Warner 1991). Three of the four rRNAs in mature ribosomes are derived from the 35S-45S rRNA precursor (pre-rRNA) transcribed by RNA polymerase I, while the fourth rRNA, 5S rRNA, is transcribed from separate genes by RNA polymerase III. The 35S-45S primary transcript is packaged into a 90S ribonucleoprotein particle (RNP), together with a subset of assembly factors and ribosomal proteins. Subsequent steps trigger folding, modification, and processing of prerRNAs and association of additional assembly factors and ribosomal proteins in 43S and 66S assembly intermediates. These pre-rRNPs undergo further maturation in the nucleolus, nucleoplasm, and then cytoplasm to form functional 40S and 60S ribosomal subunits, respectively ( Fig. 1A; FromontRacine et al. 2003;Raué 2003;Granneman and Baserga 2004). Preribosomal particles in the assembly pathway are distinguished by the presence of successive prerRNA processing intermediates (Fig. 1A). However, it is not clear into which of the consecutive preribosomes 5S rRNA and each ribosomal protein are incorporated, which assembly factors are required to recruit these molecules, or how they do so. Furthermore, the mechanisms by which constituents of nascent ribosomes facilitate folding, processing, and modification of pre-rRNAs remain elusive.5S rRNA is essential for maturation of preribosomes and for the function of mature ribosomes (Van Ryk et al. 1992;Dechampesme et al. 1999;Kiparisov et al. 2005). Steitz and coworkers defined a pathway of assembly of 5S rRNA into ribosomes in HeLa cells. Newly synthesized 5S pre-rRNA binds transiently to the La protein (Rinke and Steitz 1982;Yoo and Wolin 1994). Following 3Ј-end maturation, 5S rRNA binds to ribosomal protein rpL5, then assembles into ribosomes (...

Biogenesis of the small and large ribosomal subunits requires modification, processing, and folding of pre-rRNA to yield mature rRNA. Here, we report that efficient biogenesis of both small-and large-subunit rRNAs requires the DEAH box ATPase Prp43p, a pre-mRNA splicing factor. By steady-state analysis, a cold-sensitive prp43 mutant accumulates 35S pre-rRNA and depletes 20S, 27S, and 7S pre-rRNAs, precursors to the small-and large-subunit rRNAs. By pulse-chase analysis, the prp43 mutant is defective in the formation of 20S and 27S pre-rRNAs and in the accumulation of 18S and 25S mature rRNAs. Wild-type Prp43p immunoprecipitates pre-rRNAs and mature rRNAs, indicating a direct role in ribosome biogenesis. The Prp43p-Q423N mutant immunoprecipitates 27SA2 pre-rRNA threefold more efficiently than the wild type, suggesting a critical role for Prp43p at the earliest stages of large-subunit biogenesis. Consistent with an early role for Prp43p in ribosome biogenesis, Prp43p immunoprecipitates the majority of snoRNAs; further, compared to the wild type, the prp43 mutant generally immunoprecipitates the snoRNAs more efficiently. In the prp43 mutant, the snoRNA snR64 fails to methylate residue C 2337 in 27S pre-rRNA, suggesting a role in snoRNA function. We propose that Prp43p promotes recycling of snoRNAs and biogenesis factors during pre-rRNA processing, similar to its recycling role in pre-mRNA splicing. The dual function for Prp43p in the cell raises the possibility that ribosome biogenesis and pre-mRNA splicing may be coordinately regulated.Ribosome biogenesis is an elaborate process involving rRNA transcription, modification, processing, and folding, as well as ribonucleoprotein (RNP) assembly and export (10,20,41). Ribosome biogenesis occurs largely in the nucleolus where RNA polymerase I transcribes a large rRNA precursor (prerRNA) that is processed to mature 5.8S, 18S, and 25S rRNAs. In parallel, RNA polymerase III transcribes a precursor of mature 5S rRNA in the nucleolus. The large precursor is cleaved by Rnt1p and trimmed at the 3Ј end to yield the 35S pre-rRNA (Fig. 1A). Before processing of 35S pre-rRNA, Ͼ70 box C/D and H/ACA small nucleolar RNAs (snoRNAs) bind and modify target sequences through 2Ј-O-methylation or pseudouridinylation, respectively, in the 18S and 25S regions (6).The processing of 35S pre-rRNA initiates with cleavages at sites A0, A1, and A2, which yield the small-subunit (SSU) precursor 20S and the large-subunit precursor 27SA2 (Fig. 1B) (41). The 20S pre-rRNA is exported to the cytoplasm, where it is further modified and trimmed to yield mature 18S rRNA, the sole rRNA component of the small subunit. The 27SA2 pre-rRNA is cleaved and trimmed to generate mature 5.8S and 25S rRNAs (Fig. 1B), which then associate with 5S rRNA to form the large subunit. Although small-and large-subunit rRNAs are cotranscribed, these rRNAs are independently processed and assembled into RNPs (10).Ribosome biogenesis requires at least 18 members of the ubiquitous DExD/H box family of proteins, which generally hydro...

Dihydrouridine is a highly abundant modified nucleoside found widely in tRNAs of eubacteria, eukaryotes, and some archaea. In cytoplasmic tRNA of Saccharomyces cerevisiae, dihydrouridine occurs exclusively at positions 16, 17, 20, 20A, 20B, and 47. Here we show that the known dihydrouridine synthases Dus1p and Dus2p and two previously uncharacterized homologs, Dus3p (encoded by YLR401c) and Dus4p (YLR405w), are required for all of the dihydrouridine modification of cytoplasmic tRNAs in S. cerevisiae. We have mapped the in vivo position specificity of the four Dus proteins, by three complementary approaches: determination of the molar ratio of dihydrouridine in purified tRNAs from different dus mutants; microarray analysis of a large number of tRNAs based on differential hybridization of uridineand dihydrouridine-containing tRNAs to the complementary oligonucleotides; and the development and use of a novel dihydrouridine mapping technique, employing primer extension. We show that each of the four Dus proteins has a distinct position specificity: Dus1p for U 16 and U 17 , Dus2p for U 20 , Dus3p for U 47 , and Dus4p for U 20a and U 20b .A ubiquitous feature of tRNAs is the presence of numerous base and ribose modifications (1). More than 80 different RNA modifications have been described (2), 25 of which are found in cytoplasmic tRNAs of the yeast Saccharomyces cerevisiae; these occur at 35 positions, leading to about 11 modifications in an average yeast tRNA (3).Dihydrouridine is among the most abundant modified nucleosides found in tRNA (3); the 905 dihydrouridines found in the 561 curated tRNAs are second in number only to the 1,234 pseudouridines. Consistent with its abundance, dihydrouridine is found widely in tRNAs of eubacteria and eukaryotes (3), although it is less common in archaebacteria (4). Furthermore, dihydrouridines are found at one or more of multiple different positions in tRNA, the vast majority of which are in the D loop at positions 16, 17, 20, 20a, and 20b and at the base of the variable arm at position 47. Only in six exceptional cases is dihydrouridine found elsewhere, and in these cases it occurs at one of four other positions in the D loop (15, 17a, 19, and 21) and at position 48 in the variable arm. The persistent occurrence of dihydrouridine in these positions in so many different organisms underscores the evolutionary importance of the modification and of the sites of modification.To begin to address the roles of dihydrouridine modifications, an important first step is to decipher the substrate specificity rules for the various modified dihydrouridine residues of tRNA. The yeast S. cerevisiae is highly suitable for this analysis for three reasons. First, yeast tRNAs are modified with dihydrouridine at most of the sites of modification that have been found in characterized tRNAs. Thus, each of the six most common dihydrouridine modification sites are found in multiple yeast cytoplasmic tRNAs (Table I), and two of the five minor dihydrouridine modification sites are found in its sequen...