Analysis of subunit folding contribution of three yeast large ribosomal subunit proteins required for stabilisation and processing of intermediate nuclear rRNA precursors

In yeast and human cells many of the ribosomal proteins (r-proteins) are required for the stabilisation and productive processing of rRNA precursors. Functional coupling of r-protein assembly with the stabilisation and maturation of subunit precursors potentially promotes the production of ribosomes with defined composition. To further decipher mechanisms of such an intrinsic quality control pathway we analysed here the contribution of three yeast large ribosomal subunit r-proteins rpL2 (uL2), rpL25 (uL23) and rpL34 (eL34) for intermediate nuclear subunit folding steps. Structure models obtained from single particle cryo-electron microscopy analyses provided evidence for specific and hierarchic effects on the stable positioning and remodelling of large ribosomal subunit domains. Based on these structural and previous biochemical data we discuss possible mechanisms of r-protein dependent hierarchic domain arrangement and the resulting impact on the stability of misassembled subunits.

A structurally conserved RNA element within SARS-CoV-2 ORF1a RNA and S mRNA regulates translation in response to viral S protein-induced signaling in human lung cells.

The positive-sense, single-stranded RNA genome SARS-CoV-2 harbors functionally important cis-acting elements governing critical aspects of viral gene expression. However, insights on how these elements sense various signals from the host cell and regulate viral protein synthesis are lacking. Here, we identified two novel cis-regulatory elements in SARS-CoV-2 ORF1a and S RNAs and describe their role in translational control of SARS-CoV-2. These elements are sequence-unrelated but form conserved hairpin structures (validated by NMR) resembling G amma A ctivated I nhibitor of T ranslation (GAIT) elements that are found in a cohort of human mRNAs directing translational suppression in myeloid cells in response to IFN-γ. Our studies show that treatment of human lung cells with receptor-binding S1 subunit, S protein pseudotyped lentivirus, and S protein-containing virus-like particles triggers a signaling pathway involving DAP-kinase1 that leads to phosphorylation and release of the ribosomal protein L13a from the large ribosomal subunit. Released L13a forms a V irus A ctivated I nhibitor of T ranslation (VAIT) complex that binds to ORF1a and S VAIT elements, causing translational silencing. Translational silencing requires extracellular S protein (and its interaction with host ACE2 receptor), but not its intracellular synthesis. RNA-protein interaction analyses and in vitro translation experiments showed that GAIT and VAIT elements do not compete with each other, highlighting differences between the two pathways. Sequence alignments of SARS-CoV-2 genomes showed a high level of conservation of VAIT elements, suggesting their functional importance. This VAIT-mediated translational control mechanism of SARS-CoV-2 may provide novel targets for small molecule intervention and/or facilitate development of more effective mRNA vaccines. Importance Specific RNA elements in the genomes of RNA viruses play important roles in host-virus interaction. For SARS-CoV-2, the mechanistic insights on how these RNA elements could sense the signals from the host cell are lacking. Here we report a novel relationship between the GAIT-like SARS-CoV-2 RNA element (called VAITs) and the signal generated from the host cell. We show that for SARS-CoV-2, the interaction of spike protein with ACE2 not only serves the purpose for viral entry into the host cell, but also transduces signals that culminate into the phosphorylation and the release of L13a from the large ribosomal subunit. We also show that this event leads to the translational arrest of ORF1a and S mRNAs in a manner dependent on the structure of the RNA elements. Translational control of viral mRNA by a host-cell generated signal triggered by viral protein is a new paradigm in the host-virus relationship.

Association of snR190 snoRNA chaperone with early pre-60S particles is regulated by the RNA helicase Dbp7 in yeast

Synthesis of eukaryotic ribosomes involves the assembly and maturation of precursor particles (pre-ribosomal particles) containing ribosomal RNA (rRNA) precursors, ribosomal proteins (RPs) and a plethora of assembly factors (AFs). Formation of the earliest precursors of the 60S ribosomal subunit (pre-60S r-particle) is among the least understood stages of ribosome biogenesis. It involves the Npa1 complex, a protein module suggested to play a key role in the early structuring of the pre-rRNA. Npa1 displays genetic interactions with the DExD-box protein Dbp7 and interacts physically with the snR190 box C/D snoRNA. We show here that snR190 functions as a snoRNA chaperone, which likely cooperates with the Npa1 complex to initiate compaction of the pre-rRNA in early pre-60S r-particles. We further show that Dbp7 regulates the dynamic base-pairing between snR190 and the pre-rRNA within the earliest pre-60S r-particles, thereby participating in structuring the peptidyl transferase center (PTC) of the large ribosomal subunit.

Structural basis of ABCF-mediated resistance to pleuromutilin, lincosamide, and streptogramin A antibiotics in Gram-positive pathogens

Target protection proteins confer resistance to the host organism by directly binding to the antibiotic target. One class of such proteins are the antibiotic resistance (ARE) ATP-binding cassette (ABC) proteins of the F-subtype (ARE-ABCFs), which are widely distributed throughout Gram-positive bacteria and bind the ribosome to alleviate translational inhibition from antibiotics that target the large ribosomal subunit. Here, we present single-particle cryo-EM structures of ARE-ABCF-ribosome complexes from three Gram-positive pathogens: Enterococcus faecalis LsaA, Staphylococcus haemolyticus VgaALC and Listeria monocytogenes VgaL. Supported by extensive mutagenesis analysis, these structures enable a general model for antibiotic resistance mediated by these ARE-ABCFs to be proposed. In this model, ABCF binding to the antibiotic-stalled ribosome mediates antibiotic release via mechanistically diverse long-range conformational relays that converge on a few conserved ribosomal RNA nucleotides located at the peptidyltransferase center. These insights are important for the future development of antibiotics that overcome such target protection resistance mechanisms.

Paediatric neurosurgical implications of a ribosomopathy: illustrative case and literature review

Ribosomopathies are rare, recently defined entities. One of these, Labrune syndrome, is recognisable radiologically by its distinctive triad of leukoencephalopathy, intracranial calcifications and cysts (LCC). These cysts may have neurosurgical implications at different ages because of their progressive expansion and local mass effect. The aetiology of LCC is related to a widespread cerebral microangiopathy and is due to a genetic mutation in SNORD118, responsible for stabilisation of the large ribosomal subunit during assembly.

Analysis of subunit folding contribution of three yeast large ribosomal subunit proteins required for stabilisation and processing of intermediate nuclear rRNA precursors.

In yeast and human cells many of the ribosomal proteins (r-proteins) are required for the stabilisation and productive processing of rRNA precursors. Functional coupling of r-protein assembly with the stabilisation and maturation of subunit precursors potentially promotes the production of ribosomes with defined composition. To further decipher mechanisms of such an intrinsic quality control pathway we analysed here the contribution of three yeast large ribosomal subunit r-proteins for intermediate nuclear subunit folding steps. Structure models obtained from single particle cryo-electron microscopy analyses provided evidence for specific and hierarchic effects on the stable positioning and remodelling of large ribosomal subunit domains. Based on these structural and previous biochemical data we discuss possible mechanisms of r-protein dependent hierarchic domain arrangement and the resulting impact on the stability of misassembled subunits.

The Archaeal Elongation Factor EF-2 Induces the Release of aIF6 From 50S Ribosomal Subunit

The translation factor IF6 is a protein of about 25 kDa shared by the Archaea and the Eukarya but absent in Bacteria. It acts as a ribosome anti-association factor that binds to the large subunit preventing the joining to the small subunit. It must be released from the large ribosomal subunit to permit its entry to the translation cycle. In Eukarya, this process occurs by the coordinated action of the GTPase Efl1 and the docking protein SBDS. Archaea do not possess a homolog of the former factor while they have a homolog of SBDS. In the past, we have determined the function and ribosomal localization of the archaeal (Sulfolobus solfataricus) IF6 homolog (aIF6) highlighting its similarity to the eukaryotic counterpart. Here, we analyzed the mechanism of aIF6 release from the large ribosomal subunit. We found that, similarly to the Eukarya, the detachment of aIF6 from the 50S subunit requires a GTPase activity which involves the archaeal elongation factor 2 (aEF-2). However, the release of aIF6 from the 50S subunits does not require the archaeal homolog of SBDS, being on the contrary inhibited by its presence. Molecular modeling, using published structural data of closely related homologous proteins, elucidated the mechanistic interplay between the aIF6, aSBDS, and aEF2 on the ribosome surface. The results suggest that a conformational rearrangement of aEF2, upon GTP hydrolysis, promotes aIF6 ejection. On the other hand, aSBDS and aEF2 share the same binding site, whose occupation by SBDS prevents aEF2 binding, thereby inhibiting aIF6 release.

Paucimyces polynucleatus gen. nov, sp. nov., a novel polycentric genus of anaerobic gut fungi from the feces of a wild blackbuck antelope

The anaerobic gut fungi (AGF, phylum Neocallimastigomycota) reside in the alimentary tracts of herbivores. Multiple novel, yet-uncultured AGF taxa have recently been identified in culture-independent diversity surveys. Here, we report on the isolation and characterization of the first representative of the RH5 lineage from fecal samples of a wild blackbuck (Indian Antelope) from Sutton County, Texas, USA. The isolates displayed medium sized (2-4 mm) compact circular colonies on agar roll tubes and thin loose biofilm-like growth in liquid medium. Microscopic examination revealed monoflagellated zoospores and polycentric thalli with highly branched nucleated filamentous rhizomycelium, a growth pattern encountered in a minority of described AGF genera so far. The obtained isolates are characterized by formation of spherical vesicles at the hyphal tips from which multiple sporangia formed either directly on the spherical vesicles or at the end of sporangiophores. Phylogenetic analysis using the D1/D2 regions of the large ribosomal subunit (D/D2 LSU) and the ribosomal internal transcribed spacer 1 (ITS1) revealed sequence similarities of 93.5%, and 81.3%, respectively, to the closest cultured relatives (Orpinomyces joyonii strain D3A (D1/D2 LSU), and Joblinomyces apicalis strain GFH681 (ITS1). Substrate utilization experiments using the type strain (BB-3) demonstrated growth capabilities on a wide range of mono-, oligo-, and polysaccharides, including glucose, xylose, mannose, fructose, cellobiose, sucrose, maltose, trehalose, lactose, cellulose, xylan, starch, and raffinose. We propose accommodating these novel isolates in a new genus and species, for which the name Paucimyces polynucleatus is proposed. The type species is strain BB-3.

Innate immunity to yeast prions: Btn2p and Cur1p curing of the [URE3] prion is prevented by 60S ribosomal protein deficiency or ubiquitin/proteasome system overactivity

[URE3] is an amyloid-based prion of Ure2p, a negative regulator of poor nitrogen source catabolism in Saccharomyces cerevisiae. Overproduced Btn2p or its paralog Cur1p, in processes requiring Hsp42, cure the [URE3] prion. Btn2p cures by collecting Ure2p amyloid filaments at one place in the cell. We find that rpl4aΔ, rpl21aΔ, rpl21bΔ, rpl11bΔ and rpl16bΔ (large ribosomal subunit proteins) or ubr2Δ (ubiquitin ligase targeting Rpn4p, an activator of proteasome genes) reduce curing by overproduced Btn2p or Cur1p. Impaired curing in ubr2Δ or rpl21bΔ is restored by an rpn4Δ mutation. No effect of rps14aΔ or rps30bΔ on curing was observed, indicating that 60S subunit deficiency specifically impairs curing. Levels of Hsp42p, Sis1p or Btn3p are unchanged in rpl4aΔ, rpl21bΔ or ubr2Δ mutants. Overproduction of Cur1p or Btn2p was enhanced in rpn4Δ and hsp42Δ mutants, lower in ubr2Δ strains, and restored to above wild type levels in rpn4Δ ubr2Δ strains. As in the wild-type, Ure2N-GFP colocalizes with Btn2-RFP in rpl4aΔ, rpl21bΔ or ubr2Δ strains, but not in hsp42Δ. Btn2p/Cur1p overproduction cures [URE3] variants with low seed number, but seed number is not increased in rpl4aΔ, rpl21bΔ or ubr2Δ mutants. Knockouts of genes required for the protein sorting function of Btn2p did not affect curing of [URE3], nor did inactivation of the Hsp104 prion-curing activity. Overactivity of the ubiquitin/proteasome system, resulting from 60S subunit deficiency or ubr2Δ, may impair Cur1p and Btn2p curing of [URE3] by degrading Cur1p, Btn2p or another component of these curing systems.

Taxonomy and phylogeny of the basidiomycetous hyphomycete genus Hormomyces

The taxonomy of the genus Hormomyces, typified by Hormomyces aurantiacus, which based on circumstantial evidence was long assumed to be the hyphomycetous asexual morph of Tremella mesenterica (Tremellales, Tremellomycetes) or occasionally Dacrymyces (Dacrymycetales, Dacrymycetes), is revised. Phylogenies based on the three nuc rDNA markers [internal transcribed spacers (ITS), 28S large ribosomal subunit nrDNA (28S) and 18S small ribosomal subunit nrDNA (18S)], based on cultures from Canada and the United States, suggest that the genus is synonymous with Tulasnella (Cantharellales, Agaricomycetes) rather than Tremella or Dacrymyces. Morphological studies of 38 fungarium specimens of Hormomyces, including the type specimens of H. callorioides, H. fragiformis, H. paridiphilus and H. peniophorae and examination of the protologues of H. abieticola, H. aurantiacus and H. pezizoideus suggest that H. callorioides and H. fragiformis are conspecific with H. aurantiacus while the remaining species are unlikely to be related to Tulasnella. The conidial chains produced by H. aurantiacus are similar to monilioid cells of asexual morphs of Tulasnella species formerly referred to the genus Epulorhiza. The new combination Tulasnella aurantiaca is proposed and the species is redescribed, illustrated and compared with similar fungi. The ecological niche of T. aurantiaca and its possible relationship to orchid root endophytes is discussed. A key to asexual genera with similar conidium ontogeny to T. aurantiaca is provided.