Although the ribosome is microscopic, it is gigantic in molecular terms. The larger of its two subunits is about 50 times larger than the average enzyme. Its function is to read the genetic information encoded in messenger RNA and generate the protein molecules that those messenger RNA molecules specify. The proteins made by an organism's ribosomes are responsible for virtually all of its properties, including how it looks and behaves.

The structure of the ribosome's large subunit was determined using X-ray crystallography, a technique that can produce three-dimensional images at resolutions so high that individual atoms can be positioned. The 3,000 nucleotides of RNA in the large ribosomal subunit form a compact, complexly folded structure, and its 31 proteins permeate its RNA.

Enzymes composed entirely of protein promote virtually all chemical reactions that occur in living organisms. One of the most remarkable findings to emerge from this research is that the protein synthesis reaction that occurs on the ribosome derives from the two-thirds of its mass that is RNA, not from the one-third that is protein.

"It was suspected for many years that the RNA of the ribosome was the enzymatic component. We now know that for certain," Steitz said. "This means that in the very early days of evolution, protein synthesis evolved using RNA molecules because there were no protein molecules."

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Contact: Karen Peart
karen.peart@yale.edu
203-432-1326
Yale University
9-Aug-2000