Chromatin assembly and transcriptional cross-talk in Xenopus laevis oocyte and egg extracts

Chromatin, primarily a complex of DNA and histone proteins, is the physiological form of the genome. Chromatin is generally repressive for transcription and other information transactions that occur on DNA. A wealth of post-translational modifications on canonical histones and histone variants encode regulatory information to recruit or repel effector proteins on chromatin, promoting and further repressing transcription and thereby form the basis of epigenetic information. During metazoan oogenesis, large quantities of histone proteins are synthesized and stored in preparation for the rapid early cell cycles of development and to elicit maternal control of chromatin assembly pathways. Oocyte and egg cell-free extracts of the frog Xenopus laevis are a compelling model system for the study of chromatin assembly and transcription, precisely because they exist in an extreme state primed for rapid chromatin assembly or for transcriptional activity. We show that chromatin assembly rates are slower in the X. laevis oocyte than in egg extracts, while conversely, only oocyte extracts transcribe template plasmids. We demonstrate that rapid chromatin assembly in egg extracts represses RNA Polymerase II dependent transcription, while pre-binding of TATA-Binding Protein (TBP) to a template plasmid promotes transcription. Our experimental evidence presented here supports a model in which chromatin assembly and transcription are in competition and that the onset of zygotic genomic activation may be in part due to stable transcriptional complex assembly.