Résumé
The recent crystallographic structures of the ribosomal subunits have revealed the picture of the final product of a complex assembly process that condenses the rRNA and the ribosomal proteins into active ribosomes. The folding of rRNA is the rate-limiting step of this complicate assembly pathway. Some ribosomal proteins are required for facilitating the structural rearrangements of rRNA and avoiding the kinetic traps that frequently impede RNA folding. It is thought that the long basic r-protein extensions that penetrate deeply into the subunit cores play a key role in this process through disorder-order transitions and/or co-folding mechanisms. A current view is that such structural transitions may facilitate the proper rRNA folding. In this paper, the structures of the free and bound forms of proteins that have been experimentally found to be essential for the first steps of ribosome assembly have been compared. It is shown that the extensions of L3, L4, L20 and L22, have different structural and dynamics properties that probably correlate with different functions. This study also suggests that the specific coil-helix transition that occurs in a phylogenetically conserved cluster of basic residues of the L20 extension is strictly required for the large subunit assembly in eubacteria. In contrast, in showing that the ordering of the inner loop of L4 upon rRNA binding is not required for the assembly function of L4, this study indicates that different categories of disorder-order transitions are associated with different biological functions.