Les rétrotransposons LTR sont des éléments transposables très répandus chez les eucaryotes. Comme les rétrovirus, ils se répliquent par transcription inverse de leur ARN en ADNc, qui est intégré dans le génome hôte par leur propre intégrase (IN). Des études de séquençage à haut débit ont clairement établi que l'intégration ne se fait pas de façon aléatoire dans l'ensemble du génome de la cellule hôte. Des connaissances approfondies sur la biologie rétrovirale ont été acquises grâce à leur étude sur la levure utilisant le Ty1 LTR-retrotransposon comme modèle de travail. Le rétrotransposon Ty1 de la levure Saccharomyces cerevisiae intègre en amont des gènes de classe III, les gènes transcrits par l'ARN polymérase III (Pol III). Des données récentes ont révélé l'importance de l'AC40, une sous-unité de Pol III dans ce ciblage. Une interaction entre le Ty1 IN et l'AC40 est nécessaire pour le choix du site d'intégration des gènes Pol III. Néanmoins, le mécanisme moléculaire reste largement inconnu. Afin d'obtenir une vision globale de l'ensemble du phénomène qui se produit sur le site d'intégration, nous aimerions déterminer de manière exhaustive les protéines qui interagissent avec Ty1 IN et analyser leur rôle dans l'intégration de Ty1 et la transcription de l'ARN Pol III. Pour atteindre cet objectif, nous avons développé des approches protéomiques pour identifier de nouveaux partenaires cellulaires Ty1 intégraux. Nous avons identifié plusieurs nouveaux partenaires Ty1 IN qui semblent intéressants et leur rôle moléculaire dans la rétrotransposition de Ty1 sera étudié. Cependant, dans le cadre de mon doctorat, j'ai particulièrement travaillé à déchiffrer le rôle moléculaire de la protéine caséine kinase II dans la rétrotransposition de Ty1. / LTR-retrotransposons are widespread transposable elements in eukaryotes. Like retroviruses, they replicate by reverse transcription of their RNA into cDNA, which is integrated into the host genome by their own integrase (IN). High-throughput sequencing studies clearly established that integration does not occur randomly throughout the host-cell genome. Deep insights on retroviral biology have been gained by their study in yeast using the Ty1 LTR-retrotransposon as a working model. The Ty1 retrotransposon of the yeast Saccharomyces cerevisiae integrates upstream of class III genes, the genes transcribed by RNA polymerase III (Pol III). Recent data revealed the importance of AC40, a Pol III subunit in this targeting. An interaction between the Ty1 IN and AC40 is necessary for integration site choice at the Pol III genes. Nevertheless, the molecular mechanism remains largely unknown. To obtain a global view of the entire phenomenon that occurs on the integration site we would like to exhaustively determine the proteins that interact with Ty1 IN and analyze their role in both Ty1 integration and RNA Pol III transcription. To achieve this goal, we have developed proteomic approaches to identify new Ty1 integrase cellular partners. We have identified several novel Ty1 IN partners that seem interesting and their molecular role in Ty1 retrotransposition will be studied. However, in the tenure of my PhD, I have particularly worked to decipher the molecular role of the casein kinase II protein in Ty1 retrotransposition.

Considerable variation exists both within and between species in nuclear DNA content. Despite there being no obvious functional role for much of this DNA, many studies have reported phenotypic correlations with genome size at various taxonomic levels. This suggests that DNA plays a functional role beyond the traditionally understood mechanisms. One such example of a phenotypic correlation with DNA content is present in the genus Silene, where a negative correlation between DNA content and flower size exists within and between species. This relationship is consistent with the direction of sexual dimorphism in DNA content (caused by heteromorphic sex-chromosomes) and flower size in the most studied species in the genus: S. latifolia. This thesis takes a comparative approach between two closely related species in the genus (S. latifolia and S. marizii), which differ markedly in their nuclear DNA content, in order to investigate the nature and phenotypic impacts of variation in DNA content. A phenotypic survey from a number of S. marizii populations reveals that the pattern of DNA content variation in this species is very different to that in S. latifolia. In particular, phenotypic correlations with DNA content appear be much weaker, whilst sexual dimorphism in DNA content, when present, appears to occur in either direction. A survey of interspecific hybrids suggests that this may be due to an enlarged S. marizii X-chromosome and that DNA content in hybrids may be biased with regard to their parents. Repetitive elements may be significant constituents of plant genomes. A study of Ty1-copia class retrotransposons in the two species reveals that they are present as a large and highly heterogeneous population. Phylogenetic analysis of these elements suggests a substantial degree of genetic isolation between the two species. Finally, an assessment of the flow-cytometric method, used to estimate DNA content, reveals substantial error associated with the method, but only limited evidence for stoichiometric effects.