Adds Yonath: “We were particularly struck by the fact that this pocket is found in the ribosomes of all organisms. And this is where all the action takes place – it’s where all peptide bonds that create the chain of linked amino acids making up a protein are formed. That’s why this basic machinery seems to have persisted unchanged throughout evolution.”
Meet the protoribosome: That’s what the scientists call this pocket-like structure. They hypothesized that the protoribosome is the ancestor of all modern ribosomes, an ancient machine dating back to the prelife period. In comparison to the size of the cell, the protoribosome is rather small. It consists of RNA molecules comprising some 120 nucleotides, about 60 for each of its two semisymmetrical components, which accounts for less than 5 percent of the modern ribosome’s dimensions: some 4,500 nucleotides in bacteria and nearly 6,000 in humans
Here’s how the scenario unfolded, according to Yonath and her team:
The protoribosome came about when a bunch of RNA nucleotide chains self-assembled into two semisymmetrical walls hooked up to create a pocket. Endless other structures must have accidentally self-assembled around the same time, but the protoribosome survived, “going viral,” it seems, because it performed useful functions and, thanks to RNA’s intrinsic capabilities, it could self-replicate. When two activated amino acids happened to interact with one another within this pocket, they formed a bond, facilitated by the prevailing chemical conditions. Those amino acids may have arisen on Earth or, as some argue, landed with asteroids from outer space, but their origin is irrelevant to our story. What matters is that within the protoribosome, two activated amino acids could bind to each other. Later on, such bond making united many more amino acids, linking them into a chain.