This thesis is focused on force measurements applied to single RNA alone or associated with a protein. One of the biggest challenges arising when studying RNA comes from its structure, essentially because the interactions taking place are not reduced to the Watson-Crick base pairs and tertiary interactions are frequent. Force measurements give complementary information compared to classical bulk measurements, because they enable to probe directly the complex interactions in RNA and give an access to thermodynamical parameters which are not accessible in another way. However, an adapted and optimized setup has to be designed before accessing the features of this kind of measurement. The first part of the thesis focused on the conception of a double optical tweezers. It is adapted to the study of RNA and achieves a resolution of one base pair. This has mainly been possible thanks to the implementation of new techniques in the apparatus. Next we used our setup to work on biological issues. We probed the interaction between the ribosomal protein L20 with its target RNA in the ribosome. We show that this protein which is very important during the first steps of the ribosome assembly, stabilizes strongly its target RNA. We found its binding site with a resolution of three bases. We started the mechanical characterisation of RNA prepared to observe the activity of DEAD box helicases.
