Transcriptional Regulation of the Xenopus MyoD Gene
MyoD is one of the MRFs (Muscle Regulatory Factors) and it functions to determine the muscle cell fate. The mechanism by which MyoD regulates the muscle development program is very well understood. However, the transcriptional regulation of the MyoD gene itself has not studied in Xenopus. In this thesis, I have analyzed the transcriptional regulatory mechanism of the MyoD gene in Xenopus by different approaches, which include transgenic reporter analysis of its cis-regulatory elements of MyoD transcription and gain-of-function and loss-of-function tests of several potential regulaters. Here I showed that the expression of the XmyoD gene is controlled by a combination of induction, repression and maintenance. One activating motif, one repressing motif and one maintenance motif were found by transgenic reporter analysis. XSRF (Xenopus serum response factor) binds with the maintenance enhancer to maintain the expression of XmyoD gene. A repetitive DNA sequence was discovered in the -2.8/-2.0kb region in the XmyoD genomic sequence. The repetitive DNA sequence in the XmyoD gene locus may produce sense and anti-sense transcripts. In addition, several potential regulators have been analyzed. It has been shown that XSEB-4, a direct target of XmyoD protein, is able to induce the expression of XmyoD gene. Xenopus Ying Yang 1 (XYY1) can repress the expression of the XmyoD gene. Lef-1 is necessary and sufficient for the expression of XmyoD gene. This provides strong evidence that Lef-1 is the transcription factor of the zygotic Wnt signaling pathway that activates the expression of the XmyoD gene.
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