The Arabidopsis 2'-O-Ribose-Methylation and Pseudouridylation Landscape of rRNA in Comparison to Human and Yeast

Cryo-EM structure of wheat ribosome reveals unique features of the plant ribosomes

Plants being sessile organisms exhibit unique features in ribosomes, which might aid in rapid gene expression and regulation in response to varying environmental conditions. Here, we present high-resolution structures of the 60S and 80S ribosomes from wheat, a monocot staple crop plant (Triticum aestivum). While plant ribosomes have unique plant-specific rRNA modification (Cm1847) in the peptide exit tunnel (PET), the zinc-finger motif in eL34 is absent, and uL4 is extended, making an exclusive interaction network. We note differences in the eL15-helix 11 (25S) interaction, eL6-ES7 assembly, and certain rRNA chemical modifications between monocot and dicot ribosomes. In eukaryotes, we observe highly conserved rRNA modification (Gm75) in 5.8S rRNA and a flipped base (G1506) in PET. These features are likely involved in sensing or stabilizing nascent chain. Finally, we discuss the importance of the universal conservation of three consecutive rRNA modifications in all ribosomes for their interaction with A-site aminoacyl-tRNA.

Low dose ribosomal DNA P-loop mutation affects development and enforces autophagy in Arabidopsis

Arabidopsis contains hundreds of ribosomal DNA copies organized within the nucleolar organizing regions (NORs) in chromosomes 2 and 4. There are four major types of variants of rDNA, VAR1-4, based on the polymorphisms of 3' external transcribed sequences. The variants are known to be differentially expressed during plant development. We created a mutant by the CRISPR-Cas9-mediated excision of ~ 25 nt from predominantly NOR4 ribosomal DNA copies, obtaining mosaic mutational events on ~ 5% of all rDNA copies. The excised region consists of P-loop and Helix-82 segments of 25S rRNA. The mutation led to allelic, dosage-dependent defects marked by lateral root inhibition, reduced size, and pointy leaves, all previously observed for defective ribosomal function. The mutation in NOR4 led to dosage compensation from the NOR2 copies by elevated expression of VAR1 in mutants and further associated single-nucleotide variants, thus, resulting in altered rRNA sub-population. Furthermore, the mutants exhibited rRNA maturation defects specifically in the minor pathway typified by 32S pre-rRNA accumulation. Density-gradient fractionation and subsequent RT-PCR of rRNA analyses revealed that mutated copies were not incorporated into the translating ribosomes. The mutants in addition displayed an elevated autophagic flux as shown by the autophagic marker GFP-ATG8e, likely related to ribophagy.

High-Resolution Structure and Internal Mobility of a Plant 40S Ribosomal Subunit

Ribosome is a major part of the protein synthesis machinery, and analysis of its structure is of paramount importance. However, the structure of ribosomes from only a limited number of organisms has been resolved to date; it especially concerns plant ribosomes and ribosomal subunits. Here, we report a high-resolution cryo-electron microscopy reconstruction of the small subunit of the Triticum aestivum (common wheat) cytoplasmic ribosome. A detailed atomic model was built that includes the majority of the rRNA and some of the protein modifications. The analysis of the obtained data revealed structural peculiarities of the 40S subunit in the monocot plant ribosome. We applied the 3D Flexible Refinement approach to analyze the internal mobility of the 40S subunit and succeeded in decomposing it into four major motions, describing rotations of the head domain and a shift in the massive rRNA expansion segment. It was shown that these motions are almost uncorrelated and that the 40S subunit is flexible enough to spontaneously adopt any conformation it takes as a part of a translating ribosome or ribosomal complex. Here, we introduce the first high-resolution structure of an isolated plant 40S subunit and the first quantitative analysis of the flexibility of small ribosomal subunits, hoping that it will help in studying various aspects of ribosome functioning.

RNA pseudouridine modification in plants

Pseudouridine is one of the well-known chemical modifications in various RNA species. Current advances to detect pseudouridine show that the pseudouridine landscape is dynamic and affects multiple cellular processes. Although our understanding of this post-transcriptional modification mainly depends on yeast and human models, the recent findings provide strong evidence for the critical role of pseudouridine in plants. Here, we review the current knowledge of pseudouridine in plant RNAs, including its synthesis, degradation, regulatory mechanisms, and functions. Moreover, we propose future areas of research on pseudouridine modification in plants.

The nucleolar protein NOL12 is required for processing of large ribosomal subunit rRNA precursors in Arabidopsis

BACKGROUND

NOL12 5'-3' exoribonucleases, conserved among eukaryotes, play important roles in pre-rRNA processing, ribosome assembly and export. The most well-described yeast counterpart, Rrp17, is required for maturation of 5.8 and 25S rRNAs, whereas human hNOL12 is crucial for the separation of the large (LSU) and small (SSU) ribosome subunit rRNA precursors.

RESULTS

In this study we demonstrate that plant AtNOL12 is also involved in rRNA biogenesis, specifically in the processing of the LSU rRNA precursor, 27S pre-rRNA. Importantly, the absence of AtNOL12 alters the expression of many ribosomal protein and ribosome biogenesis genes. These changes could potentially exacerbate rRNA biogenesis defects, or, conversely, they might stem from the disturbed ribosome assembly caused by delayed pre-rRNA processing. Moreover, exposure of the nol12 mutant to stress factors, including heat and pathogen Pseudomonas syringae, enhances the observed molecular phenotypes, linking pre-rRNA processing to stress response pathways. The aberrant rRNA processing, dependent on AtNOL12, could impact ribosome function, as suggested by improved mutant resistance to ribosome-targeting antibiotics.

CONCLUSION

Despite extensive studies, the pre-rRNA processing pathway in plants remains insufficiently characterized. Our investigation reveals the involvement of AtNOL12 in the maturation of rRNA precursors, correlating this process to stress response in Arabidopsis. These findings contribute to a more comprehensive understanding of plant ribosome biogenesis.

Structure of the actively translating plant 80S ribosome at 2.2 Å resolution

In plant cells, translation occurs in three compartments: the cytosol, the plastids and the mitochondria. While the structures of the (prokaryotic-type) ribosomes in plastids and mitochondria are well characterized, high-resolution structures of the eukaryotic 80S ribosomes in the cytosol have been lacking. Here the structure of translating tobacco (Nicotiana tabacum) 80S ribosomes was solved by cryo-electron microscopy with a global resolution of 2.2 Å. The ribosome structure includes two tRNAs, decoded mRNA and the nascent peptide chain, thus providing insights into the molecular underpinnings of the cytosolic translation process in plants. The map displays conserved and plant-specific rRNA modifications and the positions of numerous ionic cofactors, and it uncovers the role of monovalent ions in the decoding centre. The model of the plant 80S ribosome enables broad phylogenetic comparisons that reveal commonalities and differences in the ribosomes of plants and those of other eukaryotes, thus putting our knowledge about eukaryotic translation on a firmer footing.

Switch them off or not: selective rRNA gene repression in grasses

Nucleolar dominance (ND) is selective epigenetic silencing of 35-48S rDNA loci. In allopolyploids, it is frequently manifested at the cytogenetic level by the inactivation of nucleolar organiser region(s) (NORs) inherited from one or several evolutionary ancestors. Grasses are ecologically and economically one of the most important land plant groups, which have frequently evolved through hybridisation and polyploidisation events. Here we review common and unique features of ND phenomena in this monocot family from cytogenetic, molecular, and genomic perspectives. We highlight recent advances achieved by using an allotetraploid model grass, Brachypodium hybridum, where ND commonly occurs at a population level, and we cover modern genomic approaches that decipher structural features of core arrays of NORs.

Dissecting Trypanosoma brucei RRP44 function in the maturation of segmented ribosomal RNA using a regulated genetic complementation system

Trypanosoma brucei belongs to a group of protozoans presenting fragmented large subunit rRNA. Its LSU rRNA equivalent to the 25S/28S rRNA of other eukaryotes is split into six fragments, requiring additional processing for removal of the extra spacer sequences. We have used a genetic complementation strategy to further investigate the T. brucei RRP44 nuclease in pre-rRNA maturation. TbRRP44 contains both a PIN and a RNB domain whose homologues are found in association with the exosome complex. We found that the exonucleolytic activity of the RNB domain as well as the physical presence of the PIN domain are essential for TbRRP44 function, while a catalytic site mutation in the PIN domain has no detectable effect on cell growth. A new endonucleolytic cleavage site in ITS1 was identified. In addition to the 5.8S rRNA 3'-end maturation, TbRRP44 is required for degradation of the excised 5'-ETS and for removal of part of ITS1 during maturation of the 18S rRNA 3'-end. TbRRP44 deficiency leads to accumulation of many LSU intermediate precursors, most of them not detected in control cells. TbRRP44 is also required for U3 snoRNA and spliced leader processing, indicating that TbRRP44 may have a wide role in RNA processing in T. brucei.

The plant epitranscriptome: revisiting pseudouridine and 2'-O-methyl RNA modifications

There is growing evidence that post-transcriptional RNA modifications are highly dynamic and can be used to improve crop production. Although more than 172 unique types of RNA modifications have been identified throughout the kingdom of life, we are yet to leverage upon the understanding to optimize RNA modifications in crops to improve productivity. The contributions of internal mRNA modifications such as N6-methyladenosine (m⁶ A) and 5-methylcytosine (m⁵ C) methylations to embryonic development, root development, leaf morphogenesis, flowering, fruit ripening and stress response are sufficiently known, but the roles of the two most abundant RNA modifications, pseudouridine (Ψ) and 2'-O-methylation (Nm), in the cell remain unclear due to insufficient advances in high-throughput technologies in plant development. Therefore, in this review, we discuss the latest methods and insights gained in mapping internal Ψ and Nm and their unique properties in plants and other organisms. In addition, we discuss the limitations that remain in high-throughput technologies for qualitative and quantitative mapping of these RNA modifications and highlight future challenges in regulating the plant epitranscriptome.