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The F-box protein Bard (CG14317) targets the Smaug RNA-binding protein for destruction during the Drosophila maternal-to-zygotic transition

ABSTRACT: During the maternal-to-zygotic transition (MZT), which encompasses the earliest stages of animal embryogenesis, a subset of maternally supplied gene products is cleared, thus permitting activation of zygotic gene expression. In the Drosophila melanogaster embryo, the RNA-binding protein Smaug plays an essential role in progression through the MZT by regulating the translational repression and degradation of a large number of maternal mRNA species. The Smaug protein itself is rapidly cleared at the end of the MZT by the Skp/Cullin/F-box (SCF) E3-ligase complex; clearance of Smaug requires zygotic transcription. Here, we show that an F-box protein, which we name Bard (encoded by CG14317), is required for degradation of Smaug. Bard is expressed zygotically and physically interacts with Smaug at the end of the MZT, coincident with binding of the maternal SCF proteins, SkpA and Cullin1, and with degradation of Smaug. We show that shRNA-mediated knock-down of Bard or deletion of the bard gene in the early embryo results in stabilization of Smaug protein, a phenotype that is rescued by transgenes expressing Bard .

INSTRUMENT(S): QE-HF

ORGANISM(S): Drosophila Melanogaster (ncbitaxon:7227)

SUBMITTER: Howard Lipshitz

PROVIDER: MSV000088199 | MassIVE | Wed Oct 06 20:11:00 BST 2021

SECONDARY ACCESSION(S): PXD028973

REPOSITORIES: MassIVE

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Project description:Background: Metazoan embryos undergo a maternal-to-zygotic transition (MZT) during which a subset of maternal gene products is eliminated and the zygotic genome becomes transcriptionally active. RNA-binding proteins (RBPs) and the microRNA-induced silencing complex (miRISC) â of which Argonaute 1 (AGO1) is a key component in Drosophila â target maternal mRNAs for degradation. The Drosophila Smaug, Brain tumor (BRAT) and Pumilio (PUM) RBPs direct the degradation of maternal mRNAs. Here we elucidate Smaugâs roles in regulation of miRNAs and miRISC during the MZT. Results: By global analysis of small RNAs at several stages during the MZT, we show that the vast majority of all miRNA species encoded by the Drosophila genome (85%) are expressed during the MZT. Whereas a subset of these miRNAs is loaded into oocytes by the mother and stays at constant levels during the MZT, dozens of miRNA species are either newly synthesized or re-expressed in the early embryo. Loss of Smaug has a profound effect on miRNAsÂ but little effect on piRNAs or siRNAs. Smaug is required for production of new miRNAs during the MZT; Smaug-bound AGO1 reflects the constellation and abundance of the miRNAs present in early embryos; and Smaug is required for the increase in AGO1 protein levels that occurs during the MZT. As a consequence of low miRISC activity in smaug mutants, maternal mRNAs that are normally targeted for degradation by zygotic miRNAs fail to be cleared. BRAT and PUM share target mRNAs with miRISC during the MZT while the miR-309 miRNA family coregulates targets of BRAT but not PUM. Conclusions: Smaug controls the MZT through direct targeting of a subset of maternal mRNAs for degradation and, indirectly, through production and function of miRNAs and miRISC, which control clearance of a distinct subset of maternal mRNAs. BRAT and/or PUM function together with miRISC during the latter process. With respect to miRISC-dependent transcript degradation, Smaug is required (1) for the synthesis of miRNAs, (2) for synthesis and stabilization of AGO1, and (3) for action of AGO1 in association with its bound miRNAs. In smaug mutants a large number of maternal mRNAs persist and the MZT fails. Examination of miRNA expresssion at different time points in wild type and smuag mutant early embryos .

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The F-box protein Bard (CG14317) targets the Smaug RNA-binding protein for destruction during the Drosophila maternal-to-zygotic transition

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