The L3 basic thumb mutants promote changes in 25S rRNA structure both locally and in elements associated with aa-tRNA and eEF2 related functions Inspection of atomic resolution ribosome structures reveals that the L3 basic thumb participates in a highly conserved set of interactions with the PTC proximal bases of Helix 73, and Helix 90, with bases on both sides of Helix 61, and with bases in a complex loop structure connecting Helices 61-64 of the LSU rRNA ( Figures 1C, 4B and D) (6, 36, (45) (46) (47) . Strikingly, while 2D maps of the LSU suggest that these structural elements are in physically separate domains from one another in (Figure 4B A-minor motif in the A-site of the peptidyltransferase center (48) . For ease of comparison, yeast 25S rRNA bases are listed with their E. coli homologs in Table 2 .