Enzyme eats self and lives to tell tale.

Enzyme eats self and lives to tell tale

A few years ago, researchers found an exception to a long-held scientific rule. They discovered RNA molecules that act as enzymes, a task previously attributed only to proteins. Also surprising was what the RNA enzymes cut: themselves.

Now, a report in the Aug. 18 NATURE describes how to design, build and test RNA enzymes, or ribozymes, targeted against particular sites on RNA strands. The technique points to promising ways of manipulating RNA for genetic engineering and gene therapy, knowledge that is especially useful because scientists are only beginning to construct or modify protein enzymes, with their more complex structures.

In order to study the basic features of a ribozyme, Jim Haseloff and Wayne Gerlach of the CSIRO Division of Plant Industry in Canberra, Australia, examined RNA of tobacco ringspot virus, which George Bruening of the University of California, Davis, had previously identified as a self-cleaver. Using genetic methods, they separated the RNA into cleaver and cleavee. Previous work had shown that a ribozyme contains a horseshoe loop with two arms extending from the loop's open bottom end, and that the strand of RNA to be cleaved is straight and straddles the ribozyme. With this rough structure in mind, the scientists examined their RNA fragments and came up with three requirements for cleavage of RNA by RNA.

First, the ribozyme's bottom end and three nucleotide bases -- the building blocks of RNA -- next to the cleavage site on the straight strand brush each other during cleavage. Second, the cutting end of the loop is the same in all ribozymes, but the end of the loop away from the cleavage site varies. Finally, the looped and straight sections both have long, flanking arms that bind together tightly during cleavage.

The scientists tested their model by building three ribozymes targeted at three sites on a strand of messenger RNA, an intermediary in the process of making protein from DNA. The model held true: Synthetic ribozymes worked as well as natural ones.

Haseloff and Gerlach say scientists may be able to use ribozymes to identify where gene transcripts lie on large fragments of RNA, and possibly even to fight disease. Ribozymes could act as "anti-genes," they explain, cleaving messenger RNA and thereby destroying the expression of unwanted genes. Says Bruening, "There are not many problems to be worked out before using this system for gene therapy."