Establishment of Zygotic Transcription and Chromatin Organization in the Early Drosophila Embryo

Chen, Kai (2013). Establishment of Zygotic Transcription and Chromatin Organization in the Early Drosophila Embryo. PhD thesis The Open University.



During animal development, the first cell divisions of a fertilized egg are under maternal control. Zygotic transcription largely begins during the midblastula transition (MBT), but some genes may be transcribed earlier. How gene activation is accomplished is poorly understood. For example, it is unclear whether any pre-pattemed markers, e.g. paused Pol II or histone modifications, are present at genes prior to activation.

In this study, I systemically investigated the dynamics of Pol II recruitment, histone modifications, and nucleosome accessibility on tightly staged Drosophila embryos to understand the establishment of zygotic transcription and chromatin organization during early embryogenesis.

Supported by evidence from histological assays, I found that the chromatin initially is loosely packed and there are no pre-recruited general transcription factors or prepatterned histone modifications in the pre-MBT embryos for the global zygotic genome activation (ZGA). In addition, widespread Pol II pausing at developmental genes is established during the MBT for later activation while massive de novo Pol II recruitment occurs. Moreover, by comparing genes activated during MBT and ~110 genes strongly occupied by Pol II in the pre-MBT embryos, I found that the lack of Pol II pausing at pre-MBT genes correlates with strong core promoters that contain the TATA-box and binding motif of Zelda, a general activator recently identified for Drosophila ZGA.

Taken together, the function of core promoters might be an underappreciated mechanism for the general regulation of the de novo establishment of chromatin structure during early Drosophila embryogenesis. It is possible that this mechanism has evolved for adapting to the quick divisions in Drosophila early embryogenesis, and this may also be true for the ZGA of other vertebrate organisms with similar dividing patterns, e. g. zebrafish and Xenopus.

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