Relics of an RNA World

Many biologists hypothesize that ribozymes are vestiges of an ancient, prebi-otic world that predated the evolution of proteins. In this "RNA world," RNAs were the catalysts of such functions as replication, cleavage, and ligation of RNA molecules. Proteins are hypothesized to have evolved later, and as they evolved they took over functions previously performed by RNA molecules. This may have happened because proteins are more versatile and efficient in their catalytic functions.

In today's world, most processing of precursor tRNA is performed by the ribozyme RNase P, as described above, but in some chloroplasts, this function is performed by a protein that apparently contains no RNA. This may be an example of the evolution of protein enzymes that replace ribozymes.

Intensive studies of ribozymes have provided rules for how they recognize their targets. Based on these rules, it has been possible to alter ribozymes to recognize and cleave new targets in RNA molecules that are normally not subject to ribozyme cleavage. These results raise the exciting possibility of using ribozymes for human therapy. For example, the abundance of disease-causing RNA molecules such as HIV, the cause of AIDS, could be reduced with artificial ribozymes. Considerable success has been achieved in testing these ribozymes in model cells. However, the biggest question remaining to be solved is how these potential "disease-fighting" ribozymes can be introduced into a patient and taken up by the appropriate cells. see also Evolution, Molecular; Proteins; RNA; RNA Processing.

Lasse Lindahl

Bibliography

Cech, T. R. "RNA as an Enzyme." Scientific American 255 (1986): 64-75.

Karp, Gerald. Cell and Molecular Biology, 3rd ed. New York: John Wiley & Sons, 2002.

complexes at which protein synthesis occurs

Ribonucleic acid (RNA) molecules, which are linear chains (or polymers) of ribonucleotides, perform a number of critical functions. Many of these functions are related to protein synthesis. Some RNA molecules bring genetic ribosomes protein-RNA information from a cell's chromosomes to its ribosomes, where proteins are assembled. Others help ribosomes translate genetic information to assemble specific sequences of amino acids.

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