473coupled timer for the MBT is further supported by the observations in Xenopus and
zebrafish that cycloheximide exposure during early cleavage stages (as opposed to
treatment one or two cycles before MBT) blocks zygotic transcription, even when
nonspecific DNA is injected to match the expected concentration of DNA at the
MBT (Lund et al. 2009 ; Lee et al. 2014 ).
As discussed above, Nanog, Pou5f3, and SoxB1 are required for zygotic gene
activation in zebrafish, and are in this sense analogous to Zelda in Drosophila.
Knockdown of all three genes causes lethality similar to treatment with α-amanitin
(Lee et al. 2013b). As these three factors are present as maternal mRNAs and are
translated prior to the MBT, their progressive accumulation could function as an
N:C independent timer. Notably, these factors are required for very early zygotic
expression of miR-430, and therefore for the regulation of maternal mRNA stability
as well as global gene activation at the MBT. Oct4 (Pou5f1) and Nanog have also
been implicated in the activation of zygotic gene expression at the two-cell stage in
mouse (Foygel et al. 2008 ; Tan et al. 2013 ). A similar role for pluripotency factors
has not yet been reported in the regulation of zygotic transcription in Xenopus.
TBP
As described above, TBP can facilitate the expression of exogenous reporter plas-
mids in cleavage stage Xenopus embryos, especially type III promoters. More gen-
erally, TBP is required for basal and active transcription and is considered an
essential component of the transcription complex (Hernandez 1993 ). TBP and
closely related factors are essential for embryonic development in Xenopus, zebraf-
ish, and C. elegans (reviewed in Lee et al. 2014 ; Bogdanovic et al. 2012 ; Veenstra
2002 ). TBP mRNA is present maternally but the protein is barely detectable in the
egg. Translation during cleavage stages leads to accumulation of TBP protein that
correlates with large-scale zygotic gene activation in frogs and mice, suggesting
new translation of TBP contributes to transcriptional activation at the MBT (Veenstra
et al. 1999 ; Bell and Scheer 1999 ; Bogdanovic et al. 2012 ; Veenstra 2002 ). Regulated
translation of maternal TBP mRNA during early development could therefore be a
component of an N:C independent timer for zygotic transcription at the MBT.
Smicl and RNAPII
Collart et al. reported an increase in C-terminal phosphorylation of RNAPII (at sites
associated with elongating RNAPII) at the MBT that was mediated by the Smad-
interacting CPSF 30-like factor (Smicl), which translocates from the cytoplasm to
the nucleus at the MBT in Xenopus. Knockdown of Smicl expression reduced the
expression of multiple genes at the MBT and delayed the degradation of several
maternal mRNAs (Collart et al. 2009 ), supporting the hypothesis that enhanced
nuclear translocation of Smicl and increased CTD phosphorylation of RNAPII
facilitates zygotic transcription at the MBT.
9 Cell Cycle Remodeling and Zygotic Gene Activation at the Midblastula Transition