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Towards a better understanding of the molecular architecture of ribosomal RNA production machinery

RNA pol I dimer is inactive and cannot bind to DNA because in this form both monomers mutually inhibit their active site. The binding Rrn3 promotes dissociation of the dimer by partnering with the dimerization interface, and allows the recruitment of the promoter of the ribosomal genes through interaction with the Core factor. Rrn3 dissociates when RNA Pol I transcribes the gene and the monomeric form released at the end of the gene tend to dimerize.

Structure of the initiation-competent RNA polymerase I and its implication for transcription.

Pilsl M(1), Crucifix C(2), Papai G(2), Krupp F(2), Steinbauer R(1), Griesenbeck J(1), Milkereit P(1), Tschochner H(1), Schultz P(2).

Nat Commun July 15, 2016

July 15, 2016

The team of Patrick Schultz at the IGBMC has unveiled the architecture of an activated form of RNA polymerase I (RNA Pol I) by cryo electron microscopy. This enzyme synthesizes a particular class of RNA which is necessary for the formation of ribosomes; the molecular machinery responsible for protein synthesis. The results are published in the journal Nature Communications, since July 15th.

RNA Pol I is the enzyme responsible for the synthesis of ribosomal RNAs. The latter can represent up to 80% of the RNA synthesized by a cell. Like the other RNA polymerases, to initiate the ARN synthesis, RNA Pol I must be positioned upstream of the gene to be transcribed on the so-called promoter region thanks to its interaction with specific transcription factors. RNA Pol I transcribes massively one single gene while the type II enzyme is recruited on more than 40,000 different genes whose. Despite many similarities, the modes of action and the control of the two enzymes are therefore very different.


Demonstrate the RNA Pol I binding mode with Rrn3 regulatory factor


To make RNA Pol I transcriptionally competent, it must interact with the protein Rrn3. The team of Patrick Schultz at the IGBMC, in collaboration with the team of Herbert Tschochner from the University of Regensburg in Germany, has analyzed the structure of a complex formed between these two partners. The researchers tried to understand the structural changes responsible for enzyme activation.


The atomic structures of both partners are known by crystallographic studies but the complex remains scarce and is difficult to obtain in vitro because its formation is regulated by a set of post-translational modifications. The German team forced the formation of this complex in yeast by overexpressing Rrn3, the minor partner. The functional complex has been purified in sufficient quantity to be observed by cryo electron microscopy.


Highlight the conformational changes required to activate the enzyme


The published work reveals the 3-D organization of the complex formed between the RNA Pol I and Rrn3 at 0.7 nm spatial resolution. At this level of detail, the secondary structure of both partners is evident allowing the understanding of how they interact with great precision.


Comparison of the new images with the atomic structure of RNA Pol I showed significant differences. The DNA-binding groove adopts a closed position and the N-terminal portion of a subunit of RNA Pol I adopts a radically different position. These changes free the channel used by the triphosphate nucleotides to access the active site of the enzyme.


All these results provide important information about the molecular architecture of the machinery of synthesis of ribosomal RNA.

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