#Hydroxymethylcytosine, well described in DNA, occurs also in RNA. Here, we show that hydroxymethylcytosine preferentially marks polyadenylated RNAs and is deposited by Tet in Drosophila. We map the transcriptome-wide hydroxymethylation landscape, revealing hydroxymethylcytosine in the transcripts of many genes, notably in coding sequences, and identify consensus sites for hydroxymethylation. We found that RNA hydroxymethylation can favor mRNA translation. Tet and hydroxymethylated RNA are found to be most abundant in the Drosophila brain, and Tet-deficient fruitflies suffer impaired brain development, accompanied by decreased RNA hydroxymethylation. This study highlights the distribution, localization, and function of cytosine hydroxymethylation and identifies central roles for this modification in Drosophila.

Secretion of antimicrobial peptides (AMPep) is a central defense mechanism used by invertebrates to combat infections. In Drosophila the synthesis of these peptides is a highly regulated process allowing their rapid release in the hemolymph upon contact with pathogens and the arrest of their production after pathogen clearance. We observed that AMPep genes have either a transient or sustained expression profile in S2 Drosophila cells treated with peptidoglycan. Moreover, AMPep genes containing AU-rich elements (ARE) in their 3-untranslated region (UTR) are subject to a post-transcriptional control affecting mRNA stability, thereby contributing to their transient expression profile. Cecropin A1 (CecA1) constitutes the prototype of this latter class of AMPeps. CecA1 mRNA bears in its 3-UTR an ARE similar to class II AREs found in several short-lived mammalian mRNAs. In response to immune deficiency cascade signaling activated by Gram-negative peptidoglycans, CecA1 mRNA is transiently stabilized and subsequently submitted to deadenylation and decay mediated by the ARE present in its 3-UTR. The functionality of CecA1 ARE relies on its ability to recruit TIS11 protein, which accelerates CecA1 mRNA deadenylation and decay. Moreover, we observed that CecA1 mRNA deadenylation is a biphasic process. Whereas early deadenylation is independent of TIS11, the later deadenylation phase depends on TIS11 and is mediated by CAF1 deadenylase. We also report that in contrast to tristetraprolin, its mammalian homolog, TIS11, is constitutively expressed in S2 cells and accumulates in cytoplasmic foci distinct from processing bodies, suggesting that the Drosophila ARE-mediated mRNA deadenylation and decay mechanism is markedly different in invertebrates and mammals.

The TIS11/tristetraprolin (TTP) CCCH tandem zinc finger proteins are major effectors in the destabilization of mRNAs bearing AU-rich elements (ARE) in their 3= untranslated regions. In this report, we demonstrate that the Drosophila melanogaster dTIS11 protein is short-lived due to its rapid ubiquitin-independent degradation by the proteasome. Our data indicate that this mechanism is tightly associated with the intrinsically unstructured, disordered N-and C-terminal domains of the protein. Furthermore, we show that TTP, the mammalian TIS11/TTP protein prototype, shares the same three-dimensional characteristics and is degraded by the same proteolytic pathway as dTIS11, thereby indicating that this mechanism has been conserved across evolution. Finally, we observed a phosphorylation-dependent inhibition of dTIS11 and TTP degradation by the proteasome in vitro, raising the possibility that such modifications directly affect proteasomal recognition for these proteins. As a group, RNAbinding proteins (RNA-BPs) have been described as enriched in intrinsically disordered regions, thus raising the possibility that the mechanism that we uncovered for TIS11/TTP turnover is widespread among other RNA-BPs.

nature structural & molecular biology advance online publication a r t i c l e sAlthough undisputable evidence has clearly demonstrated that the nuclear steps of mRNA processing are mechanistically linked to transcription, a conceptual evolution in gene regulation has come with the realization that transcription might also be functionally connected to more remote processes occurring in the cytoplasm, such as mRNA decay 1,2 . Cytoplasmic mRNA decay is initiated by deadenylation, a rate-limiting event during which the poly(A) tail of the transcript is trimmed off by the CCR4-NOT complex, the main deadenylation machinery in eukaryotes 3 . The degradation of specific mRNAs, a key process in the regulation of eukaryotic gene expression, is achieved through the recruitment of the CCR4-NOT complex by sequence-specific RNA-binding proteins (RBPs) or by the microRNA machinery 3,4 . Poly(A)-shortened mRNAs, along with factors involved in the deadenylation, decapping and mRNA-degradation machineries, accumulate in microscopic mRNA-protein complex (mRNP) aggregates called processing bodies (PBs) 5 .The idea of coupling between mRNA synthesis and degradation has recently emerged. Genome-wide expression studies in yeast have shown that mRNA synthesis and decay are mechanistically and functionally coordinated, thus supporting the existence of common molecular effectors [6][7][8][9] . In particular, the CCR4-NOT deadenylation complex was first described as a transcriptional regulator and has been implicated in initiation and elongation by RNA polymerase II 10,11 . More surprisingly, it has also been shown that degradation of yeast mRNAs is determined by cis-acting sequence elements in promoters 12,13 . These findings have led to the concept of mRNA imprinting, in which sequence-specific DNA-binding factors might orchestrate mRNA synthesis and decay. This decay would occur via loading of factors regulating cytoplasmic mRNA degradation onto the transcribing mRNA 14 . However, the identity of these DNA-binding mRNA coordinators is still obscure, and it remains to be tested whether such coupling between transcription and decay also exists in higher eukaryotes. E26 (Ets) proteins, a family of 28 helix-loop-helix transcription factors (TFs) in metazoans, are characterized by a highly conserved DNA-binding ETS domain 15 . Through this domain, Ets factors bind specific gene promoters and act as key regulators in many biological processes including cellular proliferation, apoptosis, differentiation and survival 16 . ERG, FLI1 and the more structurally divergent FEV compose the Erg subfamily of Ets factors and have been identified as driving factors in prostate cancer, Ewing's tumors and leukemias 15,17 . Using ERG as a paradigm, we sought to investigate the possibility that eukaryotic transcription factors might be directly involved in cytoplasmic mRNA decay. We demonstrate that ERG triggers degradation of mRNAs connected to Aurora signaling by recruiting RBPs and the CCR4-NOT deadenylation complex and that this activity is important fo...

TIA-1-related (TIAR) protein is a shuttling RNA-binding protein implicated in several steps of RNA metabolism. In the nucleus, TIAR contributes to alternative splicing events, whereas, in the cytoplasm, it acts as a translational repressor on specific transcripts such as adenine and uridinerich element-containing mRNAs. In addition, TIAR is involved in the general translational arrest observed in cells exposed to environmental stress. This activity is encountered by the ability of TIAR to assemble abortive pre-initiation complexes coalescing into cytoplasmic granules called stress granules. To elucidate these mechanisms of translational repression, we characterized TIAR-containing complexes by tandem affinity purification followed by MS. Amongst the identified proteins, we found the splicing factor ASF ⁄ SF2, which is also present in TIA-1 protein complexes. We show that, although mostly confined in the nuclei of normal cells, ASF ⁄ SF2 migrates into stress granules upon environmental stress. The migration of ASF ⁄ SF2 into stress granules is strictly determined both by its shuttling properties and its RNA-binding capacity. Our data also indicate that ASF ⁄ SF2 down-regulates the expression of a reporter mRNA carrying adenine and uridine-rich elements within its 3¢ UTR. Moreover, tethering of ASF ⁄ SF2 to a reporter transcript strongly reduces mRNA translation and stability. These results indicate that ASF ⁄ SF2 and TIA proteins cooperate in the regulation of mRNA metabolism in normal cells and in cells having to overcome environmental stress conditions. In addition, the present study provides new insights into the cytoplasmic function of ASF ⁄ SF2 and highlights mechanisms by which RNA-binding proteins regulate the diverse steps of RNA metabolism by subcellular relocalization upon extracellular stimuli. Abbreviations ARE, adenine and uridine-rich element; CBB, calmodulin binding