Ribosomal RNAs (rRNAs) are main effectors of messenger RNA (mRNA) decoding, peptide-bond formation, and ribosome dynamics during translation. Ribose 2'-O-methylation (2'-O-Me) is the most abundant rRNA chemical modification, and displays a complex pattern in rRNA. 2'-O-Me was shown to be essential for accurate and efficient protein synthesis in eukaryotic cells. However, whether rRNA 2'-O-Me is an adjustable feature of the human ribosome and a means of regulating ribosome function remains to be determined. Here we challenged rRNA 2'-O-Me globally by inhibiting the rRNA methyl-transferase fibrillarin in human cells. Using RiboMethSeq, a nonbiased quantitative mapping of 2'-O-Me, we identified a repertoire of 2'-O-Me sites subjected to variation and demonstrate that functional domains of ribosomes are targets of 2'-O-Me plasticity. Using the cricket paralysis virus internal ribosome entry site element, coupled to in vitro translation, we show that the intrinsic capability of ribosomes to translate mRNAs is modulated through a 2'-O-Me pattern and not by nonribosomal actors of the translational machinery. Our data establish rRNA 2'-O-Me plasticity as a mechanism providing functional specificity to human ribosomes.

Structural analyses provide new insights into the folding of the A region of the Xist RNA, which plays a crucial role in X chromosome inactivation, and its mechanism of protein recruitment.

In mammals, RNA 2'-O-methylation is a molecular signature for the discrimination of endogenous from exogenous mRNA by the cellular innate immune system 1-3 . Here we purified TRBP and its interacting partners to identify a DICERindependent TRBP complex containing FTSJ3, a putative 2'-O-methyltransferase (2'O-MTase). In vitro and ex-vivo experiments show that FTSJ3 is a 2'O-MTase that is recruited to HIV RNA through TRBP. Using RiboMethSeq analysis 4 , we identified 2'-Omethylated residues on viral genome, and found that FTSJ3 modifies HIV RNA at specific sites. HIV-1 viruses produced in FTSJ3 knock-down cells show reduced 2'-Omethylation and trigger type 1-interferons (IFN) expression in human dendritic cells through the RNA sensor MDA5. This IFN-α and IFN-β induction leads to a reduced HIV expression. Altogether, our study revealed an unexpected mechanism used by HIV to evade innate immune recognition involving the recruitment of TRBP/FTSJ3 complex to viral RNA and its 2'-O-methylation.Among post-transcriptional modifications of RNA, called epitranscriptome, 2'-Omethylation is present at the 5'-extremity of all mRNAs in mammals and internally at specific residues of tRNA, rRNA, snRNA and cellular mRNA 5 , 6 . Known for decades for its role in RNA stabilization, its key function in innate immunity has been uncovered recently 1,2 .Cellular mRNAs are capped with a 7-methyl-guanosine ( m7 G), a 5'-modification crucial for mRNA splicing, export to the cytoplasm, and efficient translation into proteins 7 . In addition, mRNAs in higher Eucaryotes acquire a second modification: a 2'-O-ribose methylation on the first (cap1) and the second transcribed nucleotide (cap2). This modification provides a molecular signature for discrimination of self from non-self mRNA. In the cell, cytoplasmic sensors, such as MDA5 and RIG-I, recognize exogenous unmodified RNAs and activate type-1 interferon production to establish an antiviral state 8,9 . To evade innate immune response, some viruses, including Flaviviruses, encode for their own viral 2'O-MTase that can add a cap1/2 on viral transcripts 10 and in the case of West Nile, Dengue and Ebola viruses can also methylate internal adenosine residues of viral genome in vitro 11,12 . In addition, in other pathogens, such as E.coli strains, tRNA 2'-O-methylation also plays an important role in immunostimulation of human dendritic cells since the loss of unique Gm18 residue in mutant TrmH E.coli strain abolishes immunosuppression 13,14 . This is due to the G18 residue of tRNA Tyr that has to be 2'-O-methylated to avoid innate sensing in human cells. In contrast to Flaviviruses, HIV does not encode a 2'O-MTase. However, FTSJ3, a putative host 2'O-MTase, was purified as a partner of TAR RNA binding protein (TRBP), a dsRNA binding