Small interfering RNAs (siRNAs), microRNAs (miRNAs), and piRNAs (piRNA) are conserved classes of small single-stranded ~21-30 nucleotide (nt) RNA guides that repress eukaryotic gene expression using distinct RNA Induced Silencing Complexes (RISCs). At its core, RISC is composed of a single-stranded small RNA guide bound to a member of the Argonaute protein family, which together bind and repress complementary target RNA. miRNAs target protein coding mRNAs—a function essential for normal development and broadly involved in pathways of human disease; small interfering RNAs (siRNA) defend against viruses, but can also be engineered to direct experimental or therapeutic gene silencing; piwi associated RNAs (piRNAs) protect germline genomes from expansion of parasitic nucleic acids such as transposons. Using the fruit fly, Drosophila melanogaster, as a model organism we seek to understand how small silencing RNAs are made and how they function. In Drosophila, miRNAs and siRNAs are proposed to have parallel, but separate biogenesis and effector machinery. miRNA duplexes are excised from imperfectly paired hairpin precursors by Dicer1 and loaded into Ago1; siRNA duplexes are hewn from perfectly paired long dsRNA by Dicer2 and loaded into Ago2. Contrary to this model we found one miRNA, miR-277, is made by Dicer1, but partitions between Ago1 and Ago2 RISCs. These two RISCs are functionally distinct—Ago2 could silence a perfectly paired target, but not a centrally bulged target; Ago1 could silence a bulged target, but not a perfect target. This was surprising since both Ago1 and Ago2 have endonucleolytic cleavage activity necessary for perfect target cleavage in vitro. Our detailed kinetic studies suggested why—Ago2 is a robust multiple turnover enzyme, but Ago1 is not. Along with a complementary in vitro study our data supports a duplex sorting mechanism in which Diced duplexes are released, and rebind to Ago1 or Ago2 loading machinery, regardless of which Dicer produced them. This allows structural information embedded in small RNA duplexes to direct small RNA loading into Ago1 and/or Ago2, resulting in distinct regulatory outputs. Small RNA sorting also has chemical consequences for the small RNA guide. Although siRNAs were presumed to have the signature 2′, 3′ hydroxyl ends left by Dicer, we found that small RNAs loaded into Ago2 or Piwi proteins, but not Ago1, are modified at their 3´ ends by the RNA 2´-O-methyltransferase DmHen1. In plants Hen1 modifies the 3´ ends all small RNAs duplexs, protecting and stabilizing them. Implying a similar function in flies, piRNAs are smaller, less abundant, and their function is perturbed in hen1 mutants. But unlike plants, small RNAs are modified as single-strands in RISC rather than as duplexes. This nicely explains why the dsRNA binding domain in plant Hen1 was discarded in animals, and why both dsRNA derived siRNAs and ssRNA derived piRNAs are modified. The recent discovery that both piRNAs and siRNAs target transposons links terminal modification and transposon silencing, suggesting that it is specialized for this purpose.

In the preceding chapters, I have discussed my doctoral research on studying the siRNA loading pathway in Drosophila using both biochemical and genetic approaches. We established a gel shift system to identify the intermediate complexes formed during siRNA loading. We detected at least three complexes, named complex B, RISC loading complex (RLC) and RISC. Using kinetic modeling, we determined that the siRNA enters complex B and RLC early during assembly when it remains double-stranded, and then matures in RISC to generate Argonaute bearing only the single-stranded guide. We further characterized the three complexes. We showed that complex B comprises Dcr-1 and Loqs, while both RLC and RISC contain Dcr-2 and R2D2. Our study suggests that the Dcr-2/R2D2 heterodimer plays a central role in RISC assembly. We observed that Dcr-1/Loqs, which function together to process pre-miRNA into mature miRNA, were also involved in siRNA loading. This was surprising, because it has been proposed that the RNAi pathway and miRNA pathway are separate and parallel, with each using a unique set of proteins to produce small RNAs, to assemble functional RNA-guided enzyme complexes, and to regulate target mRNAs. We further examined the molecular function of Dcr-1/Loqs in RNAi pathway. Our data suggest that, in vivo and in vitro, the Dcr-1/Loqs complex binds to siRNA. In vitro, the binding of the Dcr-1/Loqs complex to siRNA is the earliest detectable step in siRNA-triggered Ago2-RISC assembly. Futhermore, the binding of Dcr-1/Loqs to siRNA appears to facilitate dsRNA dicing by Dcr-2/R2D2, because the dicing activity is much lower in loqslysate than in wild type. Long inverted repeat (IR) triggered white silencing in fly eyes is an example of endogenous RNAi. Consistent with our finding that Dcr-1/Loqs function to load siRNA, less white siRNA accumulates in loqs mutant eyes compared to wild type. As a result, loqs mutants are partially defective in IR trigged whitesilencing. Our data suggest considerable functional and genetic overlap between the miRNA and siRNA pathways, with the two sharing key components previously thought to be confined to just one of the two pathways. Based on our study on siRNA loading pathway, we also elucidated the molecular function of Armitage (Armi) protein in RNAi. We showed that armi is required for RNAi. Lysates from armi mutant ovaries are defective for RNAi in vitro. Native gel analysis of protein-siRNA complexes suggests that armi mutants support early steps in the RNAi pathway, i.e., the formation of complex B and RLC, but are defective in the production of the RISC.

Transposon silencing is required to maintain genome stability. The non-coding piRNAs effectively suppress of transposon activity during germline development. In the Drosophila female germline, long precursors of piRNAs are transcribed from discrete heterochromatic clusters and then processed into primary piRNAs in the perinuclear nuage. However, the detailed mechanism of piRNA biogenesis, specifically how the nuclear and cytoplasmic processes are connected, is not well understood. The nuclear DEAD box protein UAP56 has been previously implicated in protein-coding gene transcript splicing and export. I have identified a novel function of UAP56 in piRNA biogenesis. In Drosophila egg chambers, UAP56 co-localizes with the cluster-associated HP1 variant Rhino. Nuage is a germline-specific perinuclear structure rich in piRNA biogenesis proteins, including Vasa, a DEAD box with an established role in piRNA production. Vasa-containing nuage granules localize directly across the nuclear envelope from cluster foci containing UAP56 and Rhino, and cluster transcripts immunoprecipitate with both Vasa and UAP56. Significantly, a charge-substitution mutation that alters a conserved surface residue in UAP56 disrupts co-localization with Rhino, germline piRNA production, transposon silencing, and perinuclear localization of Vasa. I therefore propose that UAP56 and Vasa function in a piRNA-processing compartment that spans the nuclear envelope.

Les topoisomérases (topos) de type IA jouent un rôle primordial dans le maintien et l’organisation du génome. Cependant, les mécanismes par lesquels elles contrôlent cette stabilité génomique sont encore à approfondir. Chez E. coli, les deux principales topoisomérases de type IA sont la topo I (codée par le gène topA) et la topo III (codée par le gène topB). Il a déjà été montré que les cellules dépourvues des topos I et III formaient de très longs filaments dans lesquels les chromosomes ne sont pas bien séparés. Comme ces défauts de ségrégation des chromosomes sont corrigés par l’inactivation de la protéine RecA qui est responsable de la recombinaison homologue, il a été émis comme hypothèse que les topoisomérases de type IA avaient un rôle dans la résolution des intermédiaires de recombinaison afin de permettre la séparation des chromosomes. D’autre part, des études réalisées dans notre laboratoire démontrent que le rôle majeur de la topoisomérase I est d’empêcher la formation des R-loops durant la transcription, surtout au niveau des opérons rrn. Ces R-loops on été récemment identifiés comme des obstacles majeurs à l’avancement des fourches de réplication, ce qui peut provoquer une instabilité génomique. Nous avons des évidences génétiques montrant qu’il en serait de même chez nos mutants topA. Tout récemment, des études ont montré le rôle majeur de certaines hélicases dans le soutien aux fourches de réplication bloquées, mais aussi une aide afin de supprimer les R-loops. Chez E. coli, ces hélicases ont été identifiées et sont DinG, Rep et UvrD. Ces hélicases jouent un rôle dans la suppression de certains obstacles à la réplication. Le but de ce projet était de vérifier l’implication de ces hélicases chez le mutant topA en utilisant une approche génétique. Étonnamment, nos résultats montrent que la délétion de certains de ces gènes d’hélicases a pour effet de corriger plutôt que d’exacerber des phénotypes du mutants topA qui sont liés à la croissance et à la morphologie des nucléoides et des cellules. Ces résultats sont interprétés à la lumière de nouvelles fonctions attribuées aux topoisomérases de types IA dans la stabilité du génome. / Type 1A topoisomerases (topos) play a vital role in the maintenance and organization of the genome. However, the mechanisms by which they control genome stability still remain to be explored. In E. coli, the two type IA topoisomerases are topo I (encoded by topA) and topo III (encoded by topB). It has been shown that cells lacking topo I and III form very long filaments in which the chromosomes are not well separated. As the chromosome segregation defects are corrected by inactivation of the RecA protein, that is responsible for homologous recombination, it has been hypothesized that type IA topoisomerases have a role in the resolution of recombination intermediates to allow chromosome segregation. On the other hand, studies in our laboratory have shown that the major role of topoisomerase I is to prevent the formation of R-loops during transcription, especially at the rrn operons. These R-loops have been recently identified as major roadblocks to the progression of replication forks, which can cause genomic instability. We have genetic evidence suggesting similar effects may occur in our topA mutants. More recently, studies have shown the important role of certain helicases in eliminating roadblocks for replication forks that could sometimes be R-loops. In E. coli, these helicases have been identified and they are DinG, Rep and UvrD. The purpose of this project was to study the roles of these helicases in our topA mutant, using a genetic approach. Surprisingly, our results show that deletions of some of these genes have the effect of correcting rather than exacerbating topA mutant phenotypes that are related to the growth and cell and nucleoid morphology. These results are interpreted in the light of new functions assigned to the type IA topoisomerases in genome stability.

A cellular pre-mRNA undergoes various post-transcriptional processing events, including capping, splicing and polyadenylation prior to nuclear export. Splicing is particularly important for mRNA nuclear export as two distinct multi-protein complexes, known as human TREX (hTREX) and the exon-junction complex (EJC), are recruited to the mRNA in a splicing-dependent manner. In contrast, a number of Kaposi's sarcoma-associated herpesvirus (KSHV) lytic mRNAs lack introns and are exported by the virus-encoded ORF57 protein. Herein we show that ORF57 binds to intronless viral mRNAs and functions to recruit the complete hTREX complex, but not the EJC, in order assemble an export component viral ribonucleoprotein particle (vRNP). The formation of this vRNP is mediated by a direct interaction between ORF57 and the hTREX export adapter protein, Aly. Aly in turn interacts directly with the DEAD-box protein UAP56, which functions as a bridge to recruit the remaining hTREX proteins to the complex. Moreover, we show that a point mutation in ORF57 which disrupts the ORF57-Aly interaction leads to a failure in the ORF57-mediated recruitment of the entire hTREX complex to the intronless viral mRNA and inhibits the mRNAs subsequent nuclear export and virus replication. Furthermore, we have utilised a trans-dominant Aly mutant to prevent the assembly of the complete ORF57-hTREX complex; this results in a vRNP consisting of viral mRNA bound to ORF57, Aly and the nuclear export factor, TAP. Strikingly, although both the export adapter Aly and the export factor TAP were present on the viral mRNP, a dramatic decrease in intronless viral mRNA export and virus replication was observed in the absence of the remaining hTREX components (UAP56 and hTHO-complex). Together, these data provide the first direct evidence that the complete hTREX complex is essential for the export of KSHV intronless mRNAs and infectious virus production.