An essential role for transcription before the MBT in Xenopus laevis

Dev Biol. 2011 Sep 15;357(2):478-91. doi: 10.1016/j.ydbio.2011.06.010. Epub 2011 Jun 29.


Most zygotic genes remain transcriptionally silent in Drosophila, Xenopus, and zebrafish embryos through multiple mitotic divisions until the midblastula transition (MBT). Several genes have been identified in each of these organisms that are transcribed before the MBT, but whether precocious expression of specific mRNAs is important for later development has not been examined in detail. Here, we identify a class of protein coding transcripts activated before the MBT by the maternal T-box factor VegT that are components of an established transcriptional regulatory network required for mesendoderm induction in Xenopus laevis, including the Nodal related ligands xnr5, xnr6, and derrière and the transcription factors bix4, and sox17α. Accumulation of phospho-Smad2, a hallmark of active Nodal signaling, at the onset of the MBT requires preMBT transcription and activity of xnr5 and xnr6. Furthermore, preMBT activation of the Nodal pathway is essential for mesendodermal gene expression and patterning of the embryo. Finally, xnr5 and xnr6 can also activate their own expression during cleavage stages, indicating that preMBT transcription contributes to a feed-forward system that allows robust activation of Nodal signaling at the MBT.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Blastula / cytology
  • Blastula / embryology*
  • Blastula / metabolism*
  • Embryo, Nonmammalian / metabolism
  • Endoderm / embryology
  • Feedback, Physiological
  • Gene Expression Regulation, Developmental
  • Nodal Protein / genetics
  • Nodal Protein / metabolism
  • Signal Transduction / genetics
  • Smad Proteins / metabolism
  • Transcription, Genetic*
  • Xenopus Proteins / genetics
  • Xenopus Proteins / metabolism
  • Xenopus laevis / embryology*
  • Xenopus laevis / genetics*


  • Nodal Protein
  • Smad Proteins
  • Xenopus Proteins