Ribosomal protein L3 in particular appears to have an important function in this process. Figure 1A shows L3 within the context of the large ribosomal subunit (LSU), including the following structural and functional elements; the peptidyltransferase center (PTC), the aa-tRNA accommodation corridor which lies in between Helix 89 and the complex Helix 90-92 structure, Helix 95 (the Sarcin/Ricin Loop or SRL), and the GTPase-associated center (GAC). The latter two structures interact with the eEF1A-aa-tRNA-GTP ternary complex and eEF2 (the eukaryotic translation elongation factor homologs of bacterial EF-Tu-aa-tRNA-GTP ternary complex and EF-G, respectively). The 3D rendering in Figure 1B shows that L3 contains a globular domain that interfaces with the solvent side of the LSU, and two structures, the N-terminal extension and a central loop that extend deep into the central core of the LSU. The central loop can be subdivided into two domains, the 'tryptophan finger' (W-finger) positioned at the tip of the central extension, and a cluster of basic amino acids that protrudes from the center of the internal loop like a thumb roughly perpendicular to the W-finger that we call the L3 'basic thumb'. Figure 1C shows that this basic thumb is nestled in the core of a cagelike structure formed by elements of three different 25S rRNA helices: H61, H73 and H90. We previously proposed a 'rocker switch' model describing how structural rearrangements of the N-terminal extension and the W-finger of L3 function to coordinate the stepwise processes of translation elongation (17) . Missing from the prior analyses was an explanation of how the movements of these extensions of L3 are communicated to functional centers of the ribosome. In this current study, analysis of the basic thumb of L3 illuminates this question. Mutagenesis of the indicated amino acids to alanines followed by genetic, biochemical and structural analyses suggests that the basic thumb acts as a molecular clamp to play a role in allosterically communicating the ribosome's tRNA occupancy status to the elongation factor binding region and the peptidyltransferase center, thus facilitating coordination of their functions through the elongation cycle.