Structure of the Ribosome at 5.5 Å Resolution and Its Interactions with Functional Ligands

Excerpt

Ribosomology as a field was launched in the late1950s, largely by the pioneering efforts of Tissières,working in the Watson laboratory at Harvard, who did thefirst physical characterizations of the E. coli ribosomeand its subunits (Tissières and Watson 1958). During thesubsequent discoveries of the genetic code and the outlines of the mechanism of translation, the ribosome wasvariously viewed as a passive surface upon which thetRNAs, mRNAs, and factors carried out the functionalsteps of protein synthesis, or, at the opposite extreme, asan enormously complex macromolecular machine whosestructural and functional elucidation would be all buthopeless. As it became clear that the ribosome was not atall inert, tremendous effort was put into the isolation andcharacterization of the ribosomal proteins, the presumeddeterminants of ribosome function. By the late 1960s,discussions over the molecular origins of life had alreadyuncovered the "ribosome paradox": How could the firstribosomes have evolved if the ribosome itself dependedon proteins for its function? Crick's solution to thischicken-or-the-egg problem was to ask whether the firstribosomes might have been made of RNA (Crick 1968).Evidence supporting this possibility began to emerge inthe early 1970s. Kethoxal modification of 30S ribosomalsubunits was shown to inactivate their ability to bindtRNA (Noller and Chaires 1972). Prior binding of tRNAspecifically protected against inactivation, suggestingthat the target of modification was the binding site itself.Reconstitution and labeling experiments showed that inactivation was due to modification of a handful of guanine bases in 16S rRNA; the proteins from the modifiedsubunits were fully active. These experiments pointed tothe possibility (but did not prove) that parts of 16S rRNAwere directly involved in binding tRNA to the ribosome...