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Dynamics of Germ Cell Specific Stable Transcriptional Complexes in Xenopus Oocytes



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Javed, Khayam 


Early animal development depends largely on the activation/expression of genes, and this often depends on the binding of transcription factors to genomic DNA sequences on which they act. This is particularly true of genes that determine cell fate; for example, Asc1-1 leads to neural differentiation and MyoD for muscle. In early development, different genes need to be activated in sequence by appropriate transcription factors. It is commonly believed that a transcription factor has only a very short dwell time of seconds or minutes on its appropriate DNA sequence. On the other hand, differentiated cell strains retain their differentiated state for many years. Does this mean that a transcription factor molecule must be exchanged for another molecule of the same kind, or could it be that a transcription factor dwell time might be significantly prolonged in stably differentiated cells, as are most non-dividing adult cell types? I have investigated the dwell time of the neurogenic transcription factor Asc1-1 by injecting mRNAs or DNAs into the non-dividing oocyte of Xenopus. Transcription factor proteins can be introduced by mRNA injection and DNA sequences to which they bind, when required; induced gene expression is monitored by expression of fluorescent reporter genes. I have used competition experiments with differently marked transcription factor proteins and DNA binding sequences to measure dwell time in living oocytes. I find that the dwell time of the factor proteins on the DNA or chromatin to which they specifically bind is much longer than is generally believed. This unexpected stability may contribute to the strength of cell differentiation in normal development. Several aspects of the regulation of transcription in non-dividing cells have been studied. Xenopus oocyte can form stable transcriptional complexes on an injected DNA template. Once formed, these transcriptional complexes stay stable for several rounds of transcription, which can last for days without letting any other DNA compete for the mRNA expression. The nature of these complexes has been thought to be tightly bound to their template DNA. However, a new mechanism governing this phenomenon of stabilised transcription in Xenopus oocyte has been proposed. According to this phenomenon, stabilised mRNA expression by a DNA template in Xenopus oocyte is established by local transcription factors' entrapment. RNA polymerase II factors have been shown to enable the formation of stabilised nuclear compartments. This finding proposes a new way of how the non-dividing cells can maintain their gene expression for a prolonged time.





Krude, Torsten
Gurdon, JB


Transcription factors, Xenopus oocytes


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Trinity College, University of Cambridge. Chief Minister Merit Scholarship, PEEF Government of Pakistan.