Funding for open access charge: Leiden Institute of Chemistry, Leiden university. A flexible loop in yeast ribosomal protein L11 coordinates P-site tRNA binding High-resolution structures reveal that yeast ribosomal protein L11 and its bacterial/archael homologs called L5 contain a highly conserved, basically charged internal loop that interacts with the peptidyl-transfer RNA (tRNA) T-loop. We call this the L11 ‘P-site loop’. Chemical protection of wild-type ribosome shows that that the P-site loop is inherently flexible, i.e. it is extended into the ribosomal P-site when this is unoccupied by tRNA, while it is retracted into the terminal loop of 25S rRNA Helix 84 when the P-site is occupied. To further analyze the function of this structure, a series of mutants within the P-site loop were created and analyzed. A mutant that favors interaction of the P-site loop with the terminal loop of Helix 84 promoted increased affinity for peptidyl-tRNA, while another that favors its extension into the ribosomal P-site had the opposite effect. The two mutants also had opposing effects on binding of aa-tRNA to the ribosomal A-site, and downstream functional effects were observed on translational fidelity, drug resistance/hypersensitivity, virus maintenance and overall cell growth. These analyses suggest that the L11 P-site loop normally helps to optimize ribosome function by monitoring the occupancy status of the ribosomal P-site. Over the past decade, atomic resolution ribosome structures have revealed the locations of critical elements. However, these static images do not reveal the dynamic movements within this complex macromolecule. The ribosome must coordinate multiple activities between spatially and functionally different sites in two subunits. These include three transfer RNA (tRNA)-binding sites, the peptidyltransferase and decoding centers and the elongation factor interacting regions. Events occurring in these regions must be carefully coordinated to assure rapid and accurate decoding of messenger RNAs (mRNAs). Current efforts in the field are focusing on determining the mechanisms by which these functional centers synchronize their actions and communicate with each other.