rDNA nascent transcripts promote a unique spatial organization during mouse early development

During the first cell cycles of the early development, the chromatin of the embryo is highly reprogrammed alongside that embryonic genome starts its own transcription. The spatial organization of the genome is a major process that contributes to regulating gene transcription in time and space, however, it is poorly studied in the context of early embryos. To study the cause and effect link between transcription and spatial organization in embryos, we focused on the ribosomal genes, that are first silent and begin to transcribe during the 2-cell stage in mouse. We demonstrated that ribosomal sequences are spatially organized in a very peculiar manner from the 2-cell to the 16-cell stage with transcription and processing of ribosomal RNAs excluding mutually. Using drugs inhibiting the RNA polymerase I, we show that this organization, totally different from somatic cells, depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors major satellite sequences transcription after the 4-cell stage. ### Competing Interest Statement The authors have declared no competing interest. * Abbreviations : 5’ETS : 5’ externally transcribed spacer ActD : Actinomycin D EGA : Embryonic Genome Activation hphCG : hours post-injection of human Chorionic Gonadotrophin H3K9ac : Histone 3 acetylated in lysine 9 H3K4me3 : Histone 3 tri-methylated in lysine 4 ITS1 : internal transcribed spacer 1 ITS2 : internal transcribed spacer 2 Major sat : mouse major satellite sequences NPB : Nucleolar Precursor Body Nopp140 : Nucleolar phosphoprotein of 140 kD mPN : maternal pronucleus pPN : paternal pronucleus PVP : Poly Vinyl Pyrrolidone RT-qPCR : Real-Time quantitative PCR RNA polymerase I : RNA pol I rDNA : ribosomal DNA genes/sequences rRNA : ribosomal RNA UBF : Upstream Binding Factor