In the first series of experiments, salt-washed wild-type and 51-4A, 54-7A, Y52Q and Y52F mutant ribosomes (chosen for structural analyses because they had the most pronounced genetic phenotypes) were treated with 1M7, an electrophile that adds an adduct onto the 2 0 OH groups of solvent exposed base sugars. Modifications were performed on salt-washed ribosomes because they represent the thermodynamic 'ground state' of the ribosome. Thus, the structural changes observed are indicative of changes in the full 'dynamic potential' of the ribosome as opposed to conformations locked in by e.g. occupation of binding sites by tRNAs or ribosome-associated factors. rRNAs were extracted, hybridized with 5 0 [ 32 P]-labeled oligonucleotide primers and reverse transcriptase primer extension reactions were performed. The products were separated through urea-acylamide denaturing gels, and visualized using a phosphorimager. 2 0 -OH ribose modification results in a strong stop 1-nt 3 0 of modified bases, and the intensity of the stops are proportional to the solvent accessibility and flexibility of riboses. Comparison of the protection patterns between wild-type and mutant ribosomes enables identification of specific bases which became protected or deprotected relative to WT.