THE DISCOVERY OF RIBOZYMES
The central role for many proteins in a cell is to catalyze chemical reactions that are essential for the cell's survival. These proteins are known as enzymes. Until relatively recently, it was thought that proteins were the only biological molecules capable of catalysis. In the early 1980s, however, research groups led by Sidney Altman and Thomas Cech independently found that RNAs can also act as catalysts for chemical reactions. This class of catalytic RNAs are known as ribozymes, and the finding earned Altman and Cech the 1989 Nobel Prize in Chemistry.
The ribozyme isolated by the Cech group, known as the Tetrahymena ribozyme, is shown in the box to the right. It acts to cut a longer strand of RNA into two smaller segments.
THE RNA WORLD HYPOTHESIS
The discovery of ribozymes supported a hypothesis, known as the RNA World Hypothesis, that earlier forms of life may have relied solely on RNA to store genetic information and to catalyze chemical reactions. This hypothesis was proposed independently by Carl Woese, Francis Crick and Leslie Orgel in the 1960s -- decades before the discovery of ribozymes -- and soon after the double-helical structure of DNA was determined. According to the RNA World Hypothesis, life later evolved to use DNA and proteins due to RNA's relative instability and poorer catalytic properties, and gradually, ribozymes became increasingly phased out.
Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme. Although the ribosome is made up of both RNA and protein components, structural and biochemical analyses revealed that the mechanisms central for translation (the process of assembling a peptide chain based on a RNA sequence) is catalyzed by RNA, not protein. This suggests that the use of RNA by early lifeforms to carry out chemical reactions preceded the use of proteins.
Rollover the image of the ribosome on the left to compare the structure with and without its protein components.
Next: Explore the role of ribozymes in protocells.