Ribosomal Structures Change When the Subunits Come Together

KEY CONCEPTS
- The head of the 30S subunit swivels around the neck when complete ribosomes are formed.
- The peptidyl transferase active site of the 50S subunit has higher activity in complete ribosomes than in individual 50S subunits.
- The interface between the 30S and 50S subunits is very rich in solvent contacts.
A body of indirect evidence suggests that the structures of the individual subunits change significantly when they join together to form a complete ribosome. Differences in the susceptibilities of the rRNAs to outside agents are one of the strongest indicators . More directly, comparisons of the highresolution crystal structures of the individual subunits with the
lower-resolution structure of the intact ribosome suggest the existence of significant differences. These ideas have been confirmed by a crystal structure of the E. coli ribosome at 3.5 Å, which furthermore identifies two different conformations of the ribosome, possibly representing different stages in translation.
The crystal contains two ribosomes per unit, each with a different conformation. The differences are due to changes in the positioning of domains within each subunit, the most important being that in one conformation the head of the small subunit has swiveled 6° around the neck region toward the E site. Also, a 6° rotation in the opposite direction is seen in the (low-resolution) structures of Thermus thermophilus ribosomes that are bound to mRNA and have tRNAs in both A and P sites, suggesting that the head may swivel overall by 12° depending on the stage of translation. The rotation of the head follows the path of tRNAs through the ribosome, raising the possibility that its swiveling controls movement of mRNA and tRNA.
The changes in conformation that occur when subunits join together are much more marked in the 30S subunit than in the 50S subunit. The changes are probably involved with controlling the position and movement of mRNA. The most significant change in the 50S subunit concerns the peptidyl transferase center. The 50S subunits are about 1,000 times less effective in catalyzing peptide bond synthesis than complete ribosomes; the reason may be a change in structure that positions the substrate more effectively in the active site in the complete ribosome.
One of the main features emerging from the structure of the complete ribosome is the very high density of solvent contacts at their interface; this may help in the making and breaking of contacts that are essential for subunit association and dissociation and may also be involved in structural changes that occur during  translocation.