La structure de la chromatine et sa compaction sont modulées par la substitution des histones conventionnelles par des variants d'histones. MacroH2A est l'un de ces variants et se singularise par sa grande taille. De nombreuses données suggèrent que macroH2A pourrait participer à l'inactivation de la transcription.<br />Par immunofluorescence, cette protéine est retrouvée accumulée sur le territoire du chromosome X inactif (Xi) chez les mammifères femelles. Néanmoins, cette association préférentielle pourrait simplement refléter la forte concentration en nucléosomes de cette région. Pour aborder le rôle de macroH2A dans le phénomène de l'inactivation du chromosome X, notre principale approche a consisté en des expériences de «ChIP-on-CHIP» sur de la chromatine native. Nos résultats montrent un enrichissement global et modeste de macroH2A sur le chromosome X femelle, excepté sur la plupart des gènes échappant à l'inactivation. <br />Nous avons souhaité nous intéresser également au rôle potentiel de macroH2A dans le mécanisme de réparation de l'ADN. En effet, il a été montré que le domaine macro est capable de lier l'ADP-ribose, un nucléotide déterminant dans de nombreux processus biologiques tels que la transcription ou la réparation. Plusieurs expériences nous ont permis de démontrer que les nucléosomes macroH2A sont associés in vivo à l'enzyme PARP-1, protéine clef de la réparation des cassures simple brin de l'ADN. La PARP-1 associée au nucléosome variant est inactive, et le traitement par H2O2 va induire son relâchement et son activation. L'absence de macroH2A conduit à une sur-activation de PARP-1, ce qui compromet sévèrement la réparation de l'ADN endommagé.

The heat shock response (HSR) is a highly conserved mechanism that enables organisms to survive environmental and pathophysiological stress. In Drosophila, the HSR is regulated by a single transcription factor, heat shock factor (HSF). During stress, HSF trimerizes and binds to over 200 loci on Drosophila polytene chromosomes with only nine mapping to major heat shock (HS) inducible gene loci. The function of HSF binding to the other sites in the genome is currently unknown. Some of these sites may contain yet unidentified “minor” HS genes. Interestingly, the binding of HSF also coincides with puff regression at some sites. Two such sites contain the major developmentally regulated genes Eip74 and Eip75: key regulators in the response to 20-hydroxyecdysone (20E), the main hormone responsible for the temporal co-ordination of post-embryonic development in Drosophila. Previous work in our and other labs indicates that the regression of non-HS puffs during the HSR is dependent on the presence of functional HSF. Using chromatin immunoprecipitation (ChIP) followed by hybridization to genome tiling arrays (Chip), I have identified 434 regions in the Drosophila Kc cell genome that are bound by HSF during HS, and have determined that 57% of these sites are located within the transcribed regions of genes. By examining the transcriptional response to HS in Kc cells and third instar larvae using expression microarrays, I found that only about 10% of all genes within 1250 bp of an HSF binding site are transcriptionally regulated by HS and many genes whose transcript levels change during HS do not appear to be near an HSF binding site. Furthermore, genes with an HSF binding site within their introns are significantly enriched (modified Fisher Exact p-value between 2.0x10-3 and 1.5x10-6) in gene ontology terms related to developmental processes and reproduction. Using expression microarray technology, I characterized the transcriptional response to 20E and its structural analog ponasterone A. I have identified multiple HSF binding sites within Eip74 and Eip75, and show that induction of the HSR correlates with repression of these genes and all other 20E-inducible genes. Taken together, this work provides a basis for further investigation into the role of HSF binding to sites not associated with HS genes and its possible function as a repressor of gene transcription during conditions of stress and as a regulator of developmental genes under stress and non-stress conditions.