maternal to zygotic transition

To begin transcription of zygotic genes, the embryo must first overcome the silencing that has been established. The cause of this silencing could be due to several factors: chromatin modifications leading to repression, lack of adequate transcription machinery, or lack of time in which significant transcription can occur due to the shortened cell cycles.{{cite journal |author=Schier AF |title=The Maternal-Zygotic Transition: Death and Birth of RNAs |journal=Science |volume=316 |pages=406–7 |year=2007 |pmid=17446392 |doi=10.1126/science.1140693 |issue=5823 | bibcode = 2007Sci...316..406S |s2cid=36999389 }} Evidence for the first method was provided by Newport and Kirschner's experiments showing that nucleocytoplasmic ratio plays a role in activating zygotic transcription.{{cite journal |vauthors=Newport J, Kirschner M |title=A major developmental transition in early Xenopus embryos: II. control of the onset of transcription. |journal=Cell |volume=30 |pages=687–96 |year=1982 |pmid= 7139712 |doi=10.1016/0092-8674(82)90273-2|issue=3|s2cid=25235449 }} They suggest that a defined amount of repressor is packaged into the egg, and that the exponential amplification of DNA at each cell cycle results in titration of the repressor at the appropriate time. Indeed, in Xenopus embryos in which excess DNA is introduced, transcription begins earlier. More recently, evidence has been shown that transcription of a subset of genes in Drosophila is delayed by one cell cycle in haploid embryos.{{cite journal |vauthors=Lu X, Li JM, Elemento O, Tavazoie S, Wieschaus EF |title=Coupling of zygotic transcription to mitotic control at the Drosophila mid-blastula transition. |journal=Development |volume=136 |pages=2101–2110 |year=2009 |pmid= 19465600 |doi=10.1242/dev.034421 |issue=12 |pmc=2685728}} The second mechanism of repression has also been addressed experimentally. Prioleau et al. show that by introducing TATA binding protein (TBP) into Xenopus oocytes, the block in transcription can be partially overcome.{{cite journal |vauthors=Prioleau MN, Huet J, Sentenac A, Mechali M |title=Competition between chromatin and transcription complex assembly regulates gene expression during early development. |journal=Cell |volume=77 |pages=439–49 |year=1994 |pmid=8181062 |doi=10.1016/0092-8674(94)90158-9 |issue=3|s2cid=1090434 }} The hypothesis that shortened cell cycles can cause repression of transcription is supported by the observation that mitosis causes transcription to cease.{{cite journal |vauthors=Shermoen AW, O'Farrell PH |title=Progression of the cell cycle through mitosis leads to abortion of nascent transcripts. |journal=Cell |volume=67 |pages=303–10 |year=1991 |pmid=1680567 |doi=10.1016/0092-8674(91)90182-X |issue=2 |pmc=2755073}}

The generally accepted mechanism for the initiation of embryonic gene regulatory networks in mammals is the occurrence of multiple waves of MZT. In mice, the first of these occurs in the zygote, where expression of a few pioneering transcription factors gradually increases the expression of target genes downstream. This induction of genes leads to a second major MZT event. {{cite journal| title=Genetic programs in human and mouse early embryos revealed by single-cell RNA sequencing|journal=Nature|volume=500|issue=7464|pages=593–597|year=2013|doi=10.1038/nature12364|pmid=23892778|last1=Xue|first1=Zhigang|last2=Huang|first2=Kevin|last3=Cai|first3=Chaochao|last4=Cai|first4=Lingbo|last5=Jiang|first5=Chun-yan|last6=Feng|first6=Yun|last7=Liu|first7=Zhenshan|last8=Zeng|first8=Qiao|last9=Cheng|first9=Liming|last10=Sun|first10=Yi E.|last11=Liu|first11=Jia-yin|last12=Horvath|first12=Steve|last13=Fan|first13=Guoping|pmc=4950944|bibcode=2013Natur.500..593X}}

To eliminate the contribution of maternal gene products to development, maternally-supplied mRNAs must be degraded in the embryo. Studies in Drosophila have shown that sequences in the 3' UTR of maternal transcripts mediate their degradation{{cite journal |vauthors=Tadros W, Lipshitz HD |title=Setting the stage for development: mRNA translation and stability during ooccyte maturation and egg activation in Drosophila |journal=Dev Dyn |volume=232 |pages=593–608 |year=2005 |pmid=15704150 |doi=10.1002/dvdy.20297 |issue=3|doi-access=free }} These sequences are recognized by regulatory proteins that cause destabilization or degradation of the transcripts. Recent studies in both zebrafish and Xenopus have found evidence of a role for microRNAs in degradation of maternal transcripts. In zebrafish, the microRNA miR-430 is expressed at the onset of zygotic transcription and targets several hundred mRNAs for deadenylation and degradation. Many of these targets are genes that are expressed maternally.{{cite journal |vauthors=Giraldez AJ, Mishima Y, Rihel J, Grocock RJ, Van Dongen S, Inoue K, Enright AJ, Schier AF |title=Zebrafish miR-430 promotes deadenylation and clearance of maternal mRNAs. |journal=Science |volume=312 |pages=75–9 |issue=5770 |year=2006 |pmid=16484454 |doi=10.1126/science.1122689|bibcode = 2006Sci...312...75G |s2cid=5529357 |doi-access=free }} Similarly, in Xenopus, the miR-430 ortholog miR-427 has been shown to target maternal mRNAs for deadenylation. Specifically, miR-427 targets include cell cycle regulators such as Cyclin A1 and Cyclin B2.{{cite journal |vauthors=Lund E, Liu M, Hartley RS, Sheets MD, Dahlberg JE |title=Deadenylation of maternal mRNAs mediated by miR-427 in Xenopus laevis embryos. |journal=RNA |volume= 15|issue=12 |pages=2351–63 |pmid=19854872 |year=2009 |doi=10.1261/rna.1882009 |pmc=2779678}}

{{Reflist|2}}

Category:Developmental biology