PhD in biological sciences offer M/F

Job Description

Thesis Subject

Ribosomes are the only molecular machines capable of synthesising proteins. Their synthesis is therefore crucial for the cell, which devotes a very significant proportion of its resources to this process. This synthesis is activated under conditions of increased demand for protein synthesis, particularly in cancers. Ribosomes consist of two subunits, a large (60S) and a small (40S) subunit, each composed of one or more ribosomal RNA (rRNA) molecules associated with numerous ribosomal proteins (RPs). The catalytic activity of ribosomes—namely, the synthesis of peptide bonds—is mediated by rRNA. This activity depends on the correct synthesis of rRNA molecules and their proper assembly with their partner RP proteins. rRNA molecules are produced from precursor molecules (pre-rRNAs), which contain additional sequences ('spacer' sequences) absent from mature rRNAs and which are progressively removed during the maturation of the pre-rRNAs. These will also undergo post-transcriptional modifications of specific nucleotides and be folded very gradually whilst associating with RPs in a specific order. These stages of maturation and assembly take place within precursor particles known as 'pre-ribosomal' particles. In addition to pre-rRNA molecules at various stages of maturation and RPs, these pre-ribosomal particles contain, at an early stage, hundreds of small nucleolar ribonucleoprotein particles (snoRNPs), as well as so-called maturation and assembly factors (AFs). snoRNPs and AFs interact with the pre-ribosomal particles and are involved, directly or indirectly, in the nucleotide modifications of pre-rRNAs, the removal of spacer sequences, the secondary and tertiary folding of pre-rRNAs, and the insertion of RPs. The majority of AFs involved in the synthesis of eukaryotic ribosomes have been identified. Many of these factors are enzymes such as helicases, kinases, GTPases, ATPases, endo- and exonucleases, methyltransferases, etc. In many cases, their substrates and precise molecular roles in ribosome synthesis remain unknown.
The thesis topic concerns the early stages of assembly and maturation of pre-ribosomal particles that are precursors to the large ribosomal subunit. These stages are among the least understood aspects of eukaryotic ribosome synthesis. This is partly due to the fact that far fewer structural data are available for the corresponding pre-ribosomal particles. Indeed, atomic-scale structures of later-stage pre-ribosomal particles have been obtained using cryo-electron microscopy (cryo-EM), enabling hypotheses regarding the mode of action of the snoRNPs and AFs they contain to be proposed and tested. No complete atomic structures are available for very early pre-ribosomal particles in the synthesis pathway of the large subunit. It is nevertheless known that these early particles contain dozens of snoRNPs and AFs, notably RNA helicases that could remodel rRNA structures or rRNA/protein interactions, and snoRNPs in complex with AFs.
The aim of the PhD project will be to elucidate, in the yeast Saccharomyces cerevisiae, the precise molecular functions in the synthesis of the large ribosomal subunit of a molecular machine consisting of a snoRNP containing the C/D-box small RNA snR190, associated with a complex of five assembly factors—known as the Npa1 complex—comprising the proteins Npa1, Npa2, Nop8, Rsa3 and the DEAD-box RNA helicase Dbp6. The studies will also be extended to investigate the roles of the homologues of these factors in human cells, where the formation of this molecular machine has yet to be demonstrated. In yeast, we have shown that this molecular machine is required for the early stages of large subunit synthesis. The aim of the PhD project will be to determine the role of this machine in the assembly of pre-ribosomal particles (i.e. the integration of RPs, AFs and snoRNPs) using quantitative mass spectrometry, in the folding of early pre-rRNA molecules using the SHAPEmap technique, and in the modification and maturation of pre-rRNA molecules using high-throughput approaches and nanopore RNA sequencing. At the same time, we will determine the structure of the yeast molecular machinery using cryo-ME, either in isolation or embedded within pre-ribosomal particles. These structures will enable us to design targeted mutations within the molecular machine that inhibit certain key interactions, in order to determine their functional consequences using the approaches listed above. Collectively, these studies should significantly enhance our understanding of the molecular mechanisms involved in the early stages of eukaryotic large subunit synthesis.

Your Work Environment

The PhD student recruited will work within a team comprising seven permanent staff members. This team is part of the MCD unit. The MCD unit focuses on fundamental research in the life sciences, centring on the mechanisms governing the organisation and function of genomes, cell division and cell mechanics, cell identity and development. The MCD unit comprises 26 teams, organising the work of over 200 people across four research programmes.

Constraints and risks

The experimental work will involve the handling of radioactive isotopes (radiation protection).

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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