Unlike the small subunit (SSU), where the four rRNA domains are largely physically distinct, the six rRNA domains of the LSU are highly intertwined (54) . With regard to the current study, the loop bounded by Helices 61-64 lie in domain IV, the PTC and Helices 90-93 and Helix 73 are in domain V, and Helices 94 and 95 are in domain IV. Previously, we demonstrated that the conformationally dynamic nature of the W-finger enables the central extension of L3 to function like a lever, and as such contribute to allosteric repositioning of rRNA structural elements (13, 17) . However, it was not clear how a small radial movement of thin, essentially planer element, could have such large and long ranging effects on rRNA structure and ribosome function. The perpendicular orientation of the basic thumb may answer this: we propose that it amplifies the action of this lever by adding three-dimensionality to the central extension in the form of a platform upon which structural elements from three different domains of 25S rRNA are anchored. In support of this, comparison of the L3 structures between EF-Tu and EF-G bound Thermus thermophilus ribosomes (55) reveals displacement of the a-carbon backbone of the W-finger and basic thumb structures by $2-3 Å , and of some individual sidechains by as much as 5 Å (Supplementary Figure S1) . This model explains how The large number of basically charged amino acids in the basic thumb enables it to participate in numerous hydrogen bonding/electrostatic interactions with rRNA bases, phosphates and riboses, functioning as a 'molecular clamp' to bridge these three domains. The current study focusing on eight amino acids in the L3 basic thumb neutralized their positive charges with alanine substitutions both singly and in selected pairs. The lethality of the R240A mutant as the sole form of L3 indicates that it may function to 'glue' the PTC proximal stem of H90 with the H61-H64 loop, and may well aid in coordinating formation of the C2876-G2922-G2951 Aminor motif that is critical for the 'induced fit' function of the peptidyltransferase center (5, 6) . Similarly, R247, which bridges Helix 61 with Helix 90 had profound effects on rRNA structure, ribosome biochemistry, translational fidelity, and cell growth, suggesting that it too is a critical bridging component between these two domains. Interestingly, the R247A/R248A double mutant was lethal, while R248A had a wild-type phenotype. This confirms the role of R247 and also indicates that the molecular defect conferred by the R247A single mutant might be partially complemented by the positively charged R248 adjacent to it. R240A and R247A appear to be the exception rather than the rule however, as the other single mutants had significantly lesser effects on these parameters. This is consistent with mutagenesis studies on other residues of L3, and with other LSU proteins and rRNA bases (11) (12) (13) (14) (15) 17, 23, (56) (57) , suggesting that the ribosome is an elegantly evolved molecular machine containing multiple levels of functional redundancy. Furthermore, all of the double mutants tested had strong effects on binding of aa-tRNA and eEF2, and on peptidyltransfer (Figure 3 ), suggesting that it multiple defects are generally required to disrupt the functionally redundant interactions between the basic thumb and the LSU rRNA. As an aside, the observation that A2926 (base paired to U2920) was protected from 1M7 modification in wild-type ribosomes when both Aand P-sites were occupied by tRNAs but deprotected when only the P-site was occupied ( Figure 4A ) is unique to yeast. To our knowledge, this has not been observed in E. coli ribosomes (51), which contains a G-C base pair (C2551-G2557) at this position, a conformational difference that suggests a potentially novel antibiotic target. Interestingly, Haloarcula marismortui appears to split the difference with a G-U base pair (U2576-G2582).