The nucleolus and rRNA genes

Responsible : Julio SAEZ VASQUEZ (DR2 CNRS)

Team :

BRANDO-GOT Sophie - Adjoint technique UPVD

COMELLA Pascale - Maître de conférences UPVD

DARRIERE Tommy - CDD chercheur CNRS

DE BURES Anne - Technicienne UPVD

JOBET Edouard - Assistant Ingénieur CNRS

SAEZ-VASQUEZ Julio - Chercheur CNRS


In our group we are interested in mechanisms controlling genome organization in plants. By genome organization we refer to 1) chromatin context (composition vs transcriptional activity) and 2) position of single or multiple copy genes (on the chromosome and in the nucleus) in active or repressed states.

We focus our research on the tandemly-repeated 45S rRNA genes which are located in two distinct chromosome territories (nucleolar organiser regions or NORs) in the nucleus. Each 45S rRNA gene transcription unit consists of sequences encoding a precursor transcript that includes the 18S, 5.8S and 25S structural rRNAs, and is separated from the adjacent gene in the array by the intergenic spacer. The nucleolus and 45S rRNA gene transcription by RNA pol I is an outstanding system to study the link between chromatin dynamics and cellular spatial organization. Indeed, permissive or transcriptionally-active rRNA genes (euchromatin) are located in the nucleolus whereas repressive or transcriptionally-inactive rRNA (heterochromatin) genes remain in the nucleoplasm, and are merely associated with the nucleolus. Thus, part of the NOR is structured as a knob of condensed chromatin, whereas the rest forms extended loops of rDNA genes from which the nucleolus originates.

The importance of 45S rDNA and the nucleolus has recently been highlighted by work on Drosophila and human cells revealing that rDNA chromatin organization can contribute to a balance between heterochromatin and euchromatin in the nucleus and that particular nuclear chromatin domains associate frequently with the nucleolus. These domains contain different classes of genes that take part in specific biological processes and might be expressed under specific cellular conditions. Thus, as the driving force for nucleolar assembly is 45S rRNA gene transcription, we can predict that rDNA dynamics might have a major impact on global nuclear gene expression. Thus rDNA and nucleolus represent a useful system to study relationship between chromatin topology and gene expression.

Using Arabidopsis thaliana as a model, we apply biochemical, cytological and functional approaches to identify molecular factors linking rDNA chromatin activity, composition and positioning in the nucleus at specific plant developmental stages and in response to environmental conditions. This study should allow us to propose an integrative and functional model to explain how nucleolar organization contributes to higher-order chromatin organization in the plant nucleus.

shema de location de la proteine d'histone

The figure shows the co-localization of histone chaperone protein AtNUC-L1 (green) with transcriptionally active (euchromatin) rRNA genes in the nucleolus of interphase cells from A. thaliana. The white signals correspond to the Nucleolar Organizer Regions (NOR2 and NOR4) containing transcriptionally silent (heterochromatin in the Knob) rRNA genes. Counterstaining with DAPI (blue) and immunodetection of histone H3K4 dimethylation (H3K4me2; red) clearly shows the nucleoplasm surrounding the nucleolus labeled with nucleolin immunostaining.

Julio Saez Vasquez (CR 1 CNRS)

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