Geminin is required for zygotic gene expression at the Xenopus mid-blastula transition.
In many organisms early development is under control of the maternal genome and zygotic gene expression is delayed until the mid-blastula transition (MBT). As zygotic transcription initiates, cell cycle checkpoints become activated and the tempo of cell division slows. The mechanisms that activate zygotic transcription at the MBT are incompletely understood, but they are of interest because they may resemble mechanisms that cause stem cells to stop dividing and terminally differentiate. The unstable regulatory protein Geminin is thought to coordinate cell division with cell differentiation. Geminin is a bi-functional protein. It prevents a second round of DNA replication during S and G2 phase by binding and inhibiting the essential replication factor Cdt1. Geminin also binds and inhibits a number of transcription factors and chromatin remodeling proteins and is thought to keep dividing cells in an undifferentiated state. We previously found that the cells of Geminin-deficient Xenopus embryos arrest in G2 phase just after the MBT then disintegrate at the onset of gastrulation. Here we report that they also fail to express most zygotic genes. The gene expression defect is cell-autonomous and is reproduced by over-expressing Cdt1 or by incubating the embryos in hydroxyurea. Geminin deficient and hydroxyurea-treated blastomeres accumulate DNA damage in the form of double stranded breaks. Bypassing the Chk1 pathway overcomes the cell cycle arrest caused by Geminin depletion but does not restore zygotic gene expression. In fact, bypassing the Chk1 pathway by itself induces double stranded breaks and abolishes zygotic transcription. We did not find evidence that Geminin has a replication-independent effect on transcription. We conclude that Geminin is required to maintain genome integrity during the rapid cleavage divisions, and that DNA damage disrupts zygotic gene transcription at the MBT, probably through activation of DNA damage checkpoint pathways.
Feinberg Cardiovascular Research Institute, Northwestern University Feinberg School of Medicine, Chicago, Illinois, United States of America.
Sarah L
SL
Kerns
Kathryn M
KM
Schultz
Kelly A
KA
Barry
Tina M
TM
Thorne
Thomas J
TJ
McGarry
D000818:Animals; D036703:Blastula / Q000378:metabolism*; D059447:Cell Cycle Checkpoints; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000378:metabolism; D004249:DNA Damage; D004268:DNA-Binding Proteins / Q000378:metabolism; D005260:Female; D064248:Geminin; D017353:Gene Deletion; D018507:Gene Expression Regulation, Developmental; D006918:Hydroxyurea / Q000494:pharmacology; D008297:Male; D009154:Mutation; D011485:Protein Binding; D020825:T-Box Domain Proteins / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014981:Xenopus / Q000196:embryology* / Q000235:genetics*; D029867:Xenopus Proteins / Q000378:metabolism; D015053:Zygote / Q000378:metabolism*
C406819:Cdt1 protein, Xenopus; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; C577998:GMNN protein, Xenopus; D064248:Geminin; D020825:T-Box Domain Proteins; C116765:TBXT protein, Xenopus; D029867:Xenopus Proteins; D006918:Hydroxyurea