## 2 tRNA Dynamics an Overview  
The potential flexible nature of the tRNA architecture was already acknowledged on the basis of the very first atomic structures. Considering the l-shape of the tRNAs, which are formed via weakly interconnected double helices (formed by the anticodon stem-D stem, and the acceptor stem-T stem, respectively), the flexibility of the tRNA molecule was assumed to be based on hinge-bending-type reconfigurations. However, experimental support for the existence of structural plasticity in the ribosome-bound forms did not come until chemical protection experiments showed that the binding of the incoming aa-tRNA in the ribosomal A site occurs in several steps via the formation of a so-called A/T state of binding [15]. In this conformation, the acceptor 3' end is still bound to elongation factor Tu (EF-Tu), whereas the ASL interacts with the mRNA. Along these lines, and confirming a previous hypothesis based on the existence of intermediate states in the movement of tRNAs through the ribosome [16], it was also shown that after the peptide bond formation, the acceptor ends of the A- and P-site tRNAs spontaneously reconfigure to form the so-called tRNA hybrid states [17]. In this configuration, the ASL of the tRNAs locate in the 30S A and P sites, while the acceptor stems are oriented towards the 50S P and E sites. Subsequently, kinetic [18] and single molecule FRET (Forster resonance energy transfer) [19] experiments have showed in increasing depth and detail that the stepwise incorporation of the aa-tRNA into the A site of the ribosome is associated with the formation of discrete structural intermediates such as the A/T state, which is characterized by a change in the conformation of the D-loop [20, 21, 22, 23, 24, 25, 26, 27]. Additional experiments have also provided evidence about the role of the formation of the hybrid state of tRNA binding for the translocation process, the rapid dynamic exchange between both states, or have highlighted the interplay of the tRNA molecules with elongation factors or other ribosomal parts/motions involved among other relevant features [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44]. In the meantime, conformational changes in the tRNA molecules have been extensively studied by computational methods, providing an extra link between structure and dynamics. Approaches such as normal mode analyses (NMA) or molecular dynamic (MD) simulations among others have allowed deeper insights into the dynamics of tRNA structures at atomic detail. Insight into tRNA flexibility was initially obtained from analysis of non ribosome-bound tRNA structures because of experimental and/or computational limitations (e.g., [9, 45, 46, 47, 48, 49, 50, 51, 52, 53]); these studies nonetheless were able to capture the D-loop and 3' CCA end's flexibility or the inter-arm angle dynamics among other relevant features (Figure 1b). The progress of computing power and capability has subsequently allowed to perform all atom, longer time simulations, as well as the simulation of tRNA's structure dynamics in complex with its various binding partners, for instance aaRS-s [54] or elongation factor EF-Tu [55]. Most relevantly, the wealth of structural data for ribosomal complexes coming from X-ray in the last decade (specific examples will be shown in the next section), have allowed to dissect the conformational space sampled by the tRNA molecules during multiple stages in the context of the entire ribosome, as well as to propose the structural and dynamic features of short-lived transition states [56, 57, 58, 59, 60, 61, 62, 63, 64]. These analyses have been a great complement to structural and dynamic fluorescence studies [65, 66]. Based on the data accumulated over years of research, the plasticity of the tRNA molecules at different stages of protein translation has been finally shown to be relevant to the whole process.