Transcription of rRNA in early mouse embryos promotes chromatin reorganization and expression of major satellite repeats.

During the first cell cycles of early development, the chromatin of the embryo is highly reprogrammed while the embryonic genome starts its own transcription. The spatial organization of the genome is an important process that contributes to regulating gene transcription in time and space. It has, however, been 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 ribosomal genes, which are silent initially but start to be transcribed in 2-cell mouse embryos. We demonstrated that ribosomal sequences and early unprocessed rRNAs are spatially organized in a very particular manner between 2-cell and 16-cell stage. By using drugs that interfere with ribosomal DNA transcription, we showed that this organization - which is totally different in somatic cells - depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors transcription of major satellite sequences once the 4-cell stage has been reached.