Premature translation of the Drosophila zygotic genome activator Zelda is not sufficient to precociously activate gene expression

Abstract

Following fertilization, the unified germ cells rapidly transition to a totipotent embryo. Maternally deposited mRNAs encode the proteins necessary for this reprogramming as the zygotic genome remains transcriptionally quiescent during the initial stages of development. The transcription factors required to activate the zygotic genome are among these maternally deposited mRNAs and are robustly translated following fertilization. In Drosophila, the mRNA encoding Zelda, the major activator of the zygotic genome, is not translated until one hour after fertilization. Here we demonstrate that zelda translation is repressed in the early embryo by the TRIM-NHL protein Brain tumor (BRAT). BRAT also regulates Zelda levels in the larval neuroblast lineage. In the embryo, BRAT-mediated translational repression is regulated by the Pan Gu (PNG) kinase, which is triggered by egg activation. The PNG kinase phosphorylates translational regulators, suggesting that PNG kinase activity alleviates translational repression of zelda by BRAT and coupling translation of zelda with that of other regulators of early embryonic development. Using the premature translation of zelda in embryos lacking BRAT activity, we showed that early translation of a zygotic genome activator is not sufficient to drive precocious gene expression. Instead, Zelda-target genes showed increased expression at the time they are normally activated. We propose that transition through early development requires the integration of multiple processes, including the slowing of the nuclear division cycle and activation of the zygotic genome. These processes are coordinately controlled by PNG kinase-mediated regulation of translation.

Chromatin accessibility is a two-tier process regulated by transcription factor pioneering and enhancer activation

“…The functional separation between pioneering and activation is consistent with previous observations in the early Drosophila embryo. Zelda unambiguously generates chromatin accessibility very early on, but is insufficient for the activation of most enhancers and functions together with patterning TFs during zygotic genome activation 40,76,108–110 . At many enhancers, Zelda is not even strictly required for enhancer activation since many patterning genes eventually become expressed in zld - embryos 38 .…”

Chromatin accessibility in the Drosophila embryo is determined by transcription factor pioneering and enhancer activation

“…Zelda unambiguously generates chromatin accessibility very early on but is insufficient for the activation of most developmental enhancers and functions together with patterning TFs during zygotic genome activation. 41 , 77 , 114 , 116 , 117 Although Zelda is essential for a small subset of genes that are expressed early and have Zelda motifs at their promoter, 38 , 80 , 93 many patterning genes such as sog do not require Zelda for activation and eventually become expressed in zld − embryos. 38 Zelda is, however, a strong potentiator of transcription.…”

Intron dynamics reveal principles of gene regulation during the maternal-to-zygotic transition

“…Smaug is a major RNA-binding protein responsible for removal of maternal transcripts. As is the case for Zelda ( Nien et al 2011 ; Eichhorn et al 2016 ; Larson et al 2022 ), the Smaug mRNA is deposited in the oocyte in a translationally repressed form, and its translation is up-regulated at egg activation ( Tadros et al 2007 ). Recognizing stem–loops in the 3′ untranslated region (3′UTR) of target transcripts, Smaug is responsible for the clearance of nearly one-third of deposited maternal mRNAs ( Tadros et al 2007 ; Chen et al 2014 ).…”

“…Because the embryo is transcriptionally silent immediately following egg activation and fertilization, these maternal gene products direct the initial events of embryogenesis ( Vastenhouw et al 2019 ). Translational control of maternal transcripts is critical for this process, and key embryonic factors are often up-regulated by a combination of changes in their poly(A)-tail length and bound regulatory factors ( Tadros et al 2007 ; Kronja et al 2014 ; Eichhorn et al 2016 ; Wang et al 2017 ; Larson et al 2022 ). As the MZT proceeds, zygotic transcription then begins with an initial, minor wave followed by more substantial transcriptional activation.…”

“…Many of these early transcribed genes contain the so-called “TAG team” motif in their promoter, which is recognized by the pioneer transcription factor Zelda ( Liang et al 2008 ). The continued activity of Zelda throughout the MZT is required for proper embryogenesis ( Liang et al 2008 ; McDaniel et al 2019 ), and its activity is restricted to this developmental window by elegant post-transcriptional gene regulation ( Harrison et al 2010 ; Nien et al 2011 ; Eichhorn et al 2016 ; Larson et al 2022 ).…”