Force measurements show that uL4 and uL24 mechanically stabilize a fragment of 23S rRNA essential for ribosome assembly

Laurent Geffroy, Thierry Bizebard, Ryo Aoyama, Takuya Ueda and Ulrich Bockelmann

In vitro reconstitution studies have shown that ribosome assembly is highly cooperative and starts with the binding of a few ribosomal (r-) proteins to rRNA. It is unknown how these early binders act. Focusing on the initial stage of the assembly of the large subunit of the E. coli ribosome, we prepared a 79 nucleotide-long region of 23S rRNA encompassing the binding sites of the early binders uL4 and uL24. Force signals are measured in a DNA/RNA dumbbell configuration with a double optical tweezers setup. The rRNA fragment was stretched until unfolded, in the absence or in the presence of the r-proteins (either uL4, uL24 or both). We show that the r-proteins uL4 and uL24 individually stabilize the rRNA fragment, both acting as molecular clamps. Interestingly, this mechanical stabilization is enhanced when both proteins are bound simultaneously. Independently, we observe a cooperative binding of uL4 and uL24 to the rRNA fragment. These two aspects of r-proteins binding both contribute to the efficient stabilization of the 3D structure of the rRNA fragment under investigation. We finally consider implications of our results for large ribosomal subunit assembly.

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