C. Elegans OMA-1 and OMA-2 are Transcriptional and Translational Repressors of Germline Fate
Ozkan, Tugba Guven
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The germline is a very specialized cell lineage for the proper transmission of genetic material through many generations, to ensure flawless perpetuation of the species and life cycles. The germline lineage is set aside as early as embryogenesis and kept quiescent until germ cells are needed for adult reproduction. During C. elegans germline development global transcription is repressed in specialized, mature diakinetic oocytes of the adult animal and transcription is reactivated as zygotic transcription in the 4-cell stage embryo but only in somatic blastomeres. Global transcription is kept repressed by PIE-1 in germline precursors beginning with 4-cell stage to protect germ lineage from inappropriate somatic differentiation pathways. During my graduate studies, I investigated the redundant roles for two CCCH type RNA binding zinc finger proteins OMA-1 and OMA-2 during C. elegans germline development and early embryogenesis. Previously, OMA proteins were shown to be required for oocyte maturation but they were not assigned any molecular functions. My thesis demonstrates transcriptional repression function of OMA proteins in newly fertilized embryos and translational repression functions during oogenesis. I showed that OMA-1/2 are redundantly required for global transcriptional repression before the onset of zygotic transcription in the 1-cell and 2-cell stages of C. elegans embryos by interacting with and sequestering in the cytoplasm TAF-4, a highly conserved essential basal transcription factor. Nuclear enrichment of TAF-4 requires interaction with another transcription factor TAF-12. OMA-1 competes with TAF-12 to interact with and change subcellular localization of TAF-4, in order to displace TAF-4 away from nuclei and prevent transcriptional initiation. I showed that interaction of OMA-1 and TAF-4 is regulated by MBK-2 phosphorylation at oocyte to embryo transition. My data suggest a model in which MBK-2 phosphorylated embryonic OMA-1 can change TAF-4 subcellular localization only in newly fertilized C. elegans embryos, not during oogenesis. When properly phosphorylated by MBK-2 kinase, ectopic OMA-1 is sufficient to repress transcription in later embryonic stages. Strikingly, reduction of oma-1/2 activities not only results in transcriptional derepression in newly fertilized embryos, but also in later germline blastomeres where wild type OMA-1 is normally absent. I show that OMA-1/2 indirectly repress global transcription in later germline blastomeres by preventing premature degradation of PIE-1 during germline development. OMA proteins protect PIE-1 and other CCCH RNA binding proteins from degradation by repressing zif-1 mRNA translation, the substrate specific binding partner for PIE-1 degradation. A zif-1 3'UTR reporter is repressed in the pachytene and proximal regions of the adult C. elegans germline, and expression of the reporter is activated in the 4-cell embryo only in anterior blastomeres, reciprocal to the PIE-1 expression pattern. I show that zif-1 3'UTR reporter is repressed in the proximal oocytes and in the pachytene region of the germline by OMA-1/2 and GLD-1 respectively. I further showed that zif-1 3'UTR reporter is kept repressed in germline blastomeres of the embryos by POS-1 and SPN-4 and its activation requires anterior cell fate determinants MEX-5/6 during embryogenesis. Contrary to the requirement for MBK-2 phosphorylated OMA-1/2 for embryonic transcriptional repression function, zif-1 3'UTR reporter repression by OMA proteins in the oocytes requires un/hypophosphorylated OMA proteins, the version of OMA-1/2 detected in the oocytes. In summary, my thesis shows that OMA-1/2 are dual function proteins redundantly required for germline development and maintenance of germline identity during oogenesis and embryonic development of C. elegans. OMA proteins are critical for the protection of CCCH type maternal proteins during oocyte development by preventing their premature proteasomal degradation through inhibiting translation of zif-1 mRNA. MBK-2 phosphorylation at the oocyte to embryo transition converts OMA proteins from oocyte translational repressors to embryonic transcriptional repressors. Phosphorylated OMA proteins can interact with TAF-4 in the newly fertilized C. elegans embryos and repress global transcription to prevent premature somatic differentiation during early stages of embryogenesis. OMA proteins protect germline identity at the level of both translational and transcriptional repression during the very critical time points of development to regulate a proper oocyte to embryonic transition.