Soutenance publique de thèse de doctorat en Sciences biologiques - François-Xavier STUBBE

Transcriptional control of a developmental transition in Caenorhabditis Elegans

Jury

Prof. Olivier DE BACKER (Département de Sciences biomédicales, UNamur), président
Prof. Nicolas GILLET (Département de Médecine vétérinaire, UNamur), secrétaire
Prof. René REZSOHAZY (Louvain Institute of Biomolecular Science and Technology, UClouvain)
Prof. Florian STEINER (Department of Molecular and Cellular Biology, UNIGE)
Prof. Shona MURPHY (Sir William Dunn School of Pathology, University of Oxford)

Résumé

Gene transcription generates gene expression patterns and allows cells to perform specialized roles within an organism to adapt to a changing environment and to maintain basic metabolic processes. Protein-coding genes are transcribed by the RNA polymerase II (Pol II). It is now established that Pol II itself is subject to many modifications that can influence how factors required for transcription and RNA processing are recruited. The largest subunit of Pol II harbors an unstructured tail-like C-terminal domain (CTD) composed of repeats of the consensus heptapeptide sequence Y₁S₂P₃T₄S₅P₆S₇. A large body of work showed that the phosphorylation status of the CTD changes in a predictable pattern as Pol II moves along the transcription unit. Pol II is recruited to transcription units with a hypo-phosphorylated Pol II and becomes heavily phosphorylated first on serine 5 (CTD-S5) during the transition from initiation to early elongation and then on serine 2 (CTD-S2) during productive elongation. While the CTD can be modified in many additional ways on all of its residues, the anti-correlated gradient of CTD-S5 and CTD-S2 phosphorylations (CTD-S5P, CTD-S2P) are the most conserved and best-characterized marks. 

 

We previously generated an analog sensitive (-as) version of the cyclin-dependent kinase 12 (CDK-12as). Inhibition of CDK-12as by a bulky ATP analog caused a significant drop in CTD-S2P, indicating that CDK-12 is the main CTD-S2 kinase. Remarkably, embryogenesis occurs normally without detectable CTD-S2P but leads to a fully penetrant L1 arrest in the F1 of inhibited worms. However, a clear mechanistic basis is still lacking. Using a straightforward suppressor screen, we identified an allele of the CTD-S5P phosphatase ssup-72 that robustly suppresses the L1 arrest induced by CDK-12as inhibition. We demonstrated that phenotypic suppression does not depend on the restoration of CTD-S2P levels and that inactivation of ssup-72 phosphatase activity alone is sufficient. Furthermore, GRO-sequencing showed that the inhibition of CDK-12 globally affects transcription elongation but is not causal to L1 arrest. When CDK-12 is inhibited, Pol II tends to terminate early, which is counteracted in SSUP-72 mutants. Our data indicate a novel link between CDK-12 and SSUP-72 in transcription termination and its impact in development.

 

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