7-Methylguanosine 5′ cap on mRNA is necessary for efficient protein expression in vitro and in vivo. Recent studies revealed structural diversity of endogenous mRNA caps, which carry different 5′-terminal nucleotides and additional methylations (2′-O-methylation and m6A). Currently available 5′-capping methods do not address this diversity. We report trinucleotide 5′ cap analogs (m7GpppN(m)pG), which are utilized by RNA polymerase T7 to initiate transcription from templates carrying Φ6.5 promoter and enable production of mRNAs differing in the identity of the first transcribed nucleotide (N = A, m6A, G, C, U) and its methylation status (±2′-O-methylation). HPLC-purified mRNAs carrying these 5′ caps were used to study protein expression in three mammalian cell lines (3T3-L1, HeLa and JAWS II). The highest expression was observed for mRNAs carrying 5′-terminal A/Am and m6Am, whereas the lowest was observed for G and Gm. The mRNAs carrying 2′-O-methyl at the first transcribed nucleotide (cap 1) had significantly higher expression than unmethylated counterparts (cap 0) only in JAWS II dendritic cells. Further experiments indicated that the mRNA expression characteristic does not correlate with affinity for translation initiation factor 4E or in vitro susceptibility to decapping, but instead depends on mRNA purity and the immune state of the cells.

Edited by Michael IbbaProper regulation of ribosome biosynthesis is mandatory for cellular adaptation, growth and proliferation. Ribosome biogenesis is the most energetically demanding cellular process, which requires tight control. Abnormalities in ribosome production have severe consequences, including developmental defects in plants and genetic diseases (ribosomopathies) in humans. One of the processes occurring during eukaryotic ribosome biogenesis is processing of the ribosomal RNA precursor molecule (pre-rRNA), synthesized by RNA polymerase I, into mature rRNAs. It must not only be accurate but must also be precisely coordinated with other phenomena leading to the synthesis of functional ribosomes: RNA modification, RNA folding, assembly with ribosomal proteins and nucleocytoplasmic RNP export. A multitude of ribosome biogenesis factors ensure that these events take place in a correct temporal order. Among them are endo-and exoribonucleases involved in pre-rRNA processing. Here, we thoroughly present a wide spectrum of ribonucleases participating in rRNA maturation, focusing on their biochemical properties, regulatory mechanisms and substrate specificity. We also discuss cooperation between various ribonucleolytic activities in particular stages of pre-rRNA processing, delineating major similarities and differences between three representative groups of eukaryotes: yeast, plants and humans.Keywords: endoribonuclease; exoribonuclease; pre-rRNA processing; ribosome biogenesis; RNA quality control; RNA trimming Ribosome biosynthesis is a multistep process that includes several tightly controlled events -ribosomal DNA (rDNA) transcription, processing of rRNA precursors into mature molecules, accompanied by binding of ribosome biogenesis factors (RBFs) and ribosomal proteins (RPs), and the final assembly of all Abbreviations CD, catalytic domain; dsRBD, double-stranded RNA-binding domain; ETS, external transcribed spacer; ITS, internal transcribed spacer; LSU, large ribosome subunit (60S); miRNA, microRNA; mRNA, messenger RNA; MRP RNA, noncoding RNA component of the RNase MRP; mTOR, mammalian target of rapamycin; nt(s), nucleotide(s); PIN domain, PilT N-terminal domain; Pol I/II/III, RNA polymerase I/II/III; prerRNA, ribosomal RNA precursor; RBF, ribosome biogenesis factor; RMRP, RNase MRP (mitochondrial RNA processing ribonuclease); RNase, ribonuclease; RNB domain, RNase II domain; RNP, ribonucleoprotein; RP, ribosomal protein; rRNA, ribosomal RNA; siRNA, short interfering RNA; snoRNA, small nucleolar RNA; snoRNP, small nucleolar ribonucleoprotein; snRNA, small nuclear RNA; SSU, small ribosome subunit (40S); TRAMP4 or TRAMP5, Trf4/Air/Mtr4 or Trf5/Air/Mtr4 polyadenylation complex; tRNA, transfer RNA.

Sm-like (Lsm) proteins have been identified in all organisms and are related to RNA metabolism. Here, we report that Arabidopsis nuclear AtLSM8 protein, as well as AtLSM5, which localizes to both the cytoplasm and nucleus, function in pre-mRNA splicing, while AtLSM5 and the exclusively cytoplasmic AtLSM1 contribute to 5′–3′ mRNA decay. In lsm8 and sad1/lsm5 mutants, U6 small nuclear RNA (snRNA) was reduced and unspliced mRNA precursors accumulated, whereas mRNA stability was mainly affected in plants lacking AtLSM1 and AtLSM5. Some of the mRNAs affected in lsm1a lsm1b and sad1/lsm5 plants were also substrates of the cytoplasmic 5′–3′ exonuclease AtXRN4 and of the decapping enzyme AtDCP2. Surprisingly, a subset of substrates was also stabilized in the mutant lacking AtLSM8, which supports the notion that plant mRNAs are actively degraded in the nucleus. Localization of LSM components, purification of LSM-interacting proteins as well as functional analyses strongly suggest that at least two LSM complexes with conserved activities in RNA metabolism, AtLSM1-7 and AtLSM2-8, exist also in plants.

The 5′ cap consists of 7-methyl­guanosine (m7G) linked by a 5′–5′-triphosphate bridge to messenger RNA (mRNA) and acts as the master regulator of mRNA turnover and translation initiation in eukaryotes. Cap analogues that influence mRNA translation and turnover (either as small molecules or as part of an RNA transcript) are valuable tools for studying gene expression, which is often also of therapeutic relevance. Here, we synthesized a series of 15 dinucleotide cap (m7GpppG) analogues containing a 5′-phosphoro­thiolate (5′-PSL) moiety (i.e., an O-to-S substitution within the 5′-phospho­ester) and studied their biological properties in the context of three major cap-binding proteins: translation initiation factor 4E (eIF4E) and two decapping enzymes, DcpS and Dcp2. While the 5′-PSL moiety was neutral or slightly stabilizing for cap interactions with eIF4E, it significantly influenced susceptibility to decapping. Replacing the γ-phospho­ester with the 5′-PSL moiety (γ-PSL) prevented β-γ-pyrophosphate bond cleavage by DcpS and conferred strong inhibitory properties. Combining the γ-PSL moiety with α-PSL and β-phosphoro­thioate (PS) moiety afforded first cap-derived hDcpS inhibitor with low nanomolar potency. Susceptibility to Dcp2 and translational properties were studied after incorporation of the new analogues into mRNA transcripts by RNA polymerase. Transcripts containing the γ-PSL moiety were resistant to cleavage by Dcp2. Surprisingly, superior translational properties were observed for mRNAs containing the α-PSL moiety, which were Dcp2-susceptible. The overall protein expression measured in HeLa cells for this mRNA was comparable to mRNA capped with the translation augmenting β-PS analogue reported previously. Overall, our study highlights 5′-PSL as a synthetically accessible cap modification, which, depending on the substitution site, can either reduce susceptibility to decapping or confer superior translational properties on the mRNA. The 5′-PSL-analogues may find application as reagents for the preparation of efficiently expressed mRNA or for investigation of the role of decapping enzymes in mRNA processing or neuromuscular disorders associated with decapping.

SUMMARYThe biosynthesis of ribosomal RNA and its incorporation into functional ribosomes is an essential and intricate process that includes production of mature ribosomal RNA from large precursors. Here, we analyse the contribution of the plant exosome and its co-factors to processing and degradation of 18S pre-RNAs in Arabidopsis thaliana. Our data show that, unlike in yeast and humans, an RRP6 homologue, the nucleolar exoribonuclease RRP6L2, and the exosome complex, together with RRP44, function in two distinct steps of pre-18S rRNA processing or degradation in Arabidopsis. In addition, we identify TRL (TRF4/5-like) as the terminal nucleotidyltransferase that is mainly responsible for oligoadenylation of rRNA precursors in Arabidopsis. We show that TRL is required for efficient elimination of the excised 5 0 external transcribed spacer and of 18S maturation intermediates that escaped 5 0 processing. Our data also suggest involvement of additional nucleotidyltransferases, including terminal uridylyltransferase(s), in modifying rRNA processing intermediates in plants.