New and primordial role for ribozymes?

New and priomordial role for ribozymes?

According to some origin-of-life theories, the DNA-based genetic machinery found in almost all modern organisms arose from an ancient "RNA world" (SN: 10/7/89, p.229). Evidence for this scenario stems mainly from the recent discovery of ribozymes: RNA molecules capable of carrying out biochemical tasks that scientists once believed only protein enzymes could perform.

Unlike proteins, which can act as catalysts to direct the cell's complex workings, all ribozymes found so far chemically transformed only RNA (ribonucleic acid). But a new report hints ribozymes may be capable of altering non-RNA molecules as well. If confirmed, the work would enlarge the known range of reactions in which ribozymes participate and strengthen support for the concept of a primitive RNA world.

In the Nov. 27 BIOCHEMISTRY, Japanese scientists report that RNA from a yeast catalyzed the transfer of electrons from one non-RNA molecule to another. Such electron-shuffling processes, known as "redox" reactions, play a crucial role in the metalbolism of organisms. Researchers at Mitsubishi Kasei Institute of Life Sciences in Tokyo and Tohoku University in Sendai identified the catalytic agent as 5-hydroxycytidine, a modified form of one of the four basic nucleoside building blocks making up every RNA chain.

The finding "indicates new possibilities for RNA as a living molecule," coauthor Hiroshi Yanagawa told SCIENCE NEWS. That an RNA can assist in redox reactions might mean that before proteins evolved, primordial RNAs served as catalysts driving life-sustaining metabolic processes, the Mitsubishi Kasei scientist suggests. His team proposes that 5-hydroxycytidine may be a vestige of such ancient RNAs.

But others say these claims lack supporting evidence. "Linking [5-hydroxycytidine] to an RNA world is a little premature" because such modified nucleosides are not necessarily "old" in an evolutionary sense, says biochemist Andrew D. Ellington at Massachusetts General Hospital in Boston. Moreover, he notes, scientists have long known that molecules containing modified nucleosides can help drive metabolic reactions.

Thomas R. Cech of the University of Colorado in Boulder also points out that the enzyme-like agent s only a modified nucleoside -- not a "proper ribozyme," or folded chain of RNA nucleosides that binds a substance to foster a specific biochemical reaction. The Japanese team has yet to show that a yeast RNA containing the isolated agent meets this definition, suggests Cech, the 1989 Nobel laureate who coined the term "ribozyme."

However, Cech acknowledges, the new work does carry important implications. Until now, research has indicated that catalytic RNAs perform only a "limited range of reactions" -- making and breaking bonds in RNA. But if a modified nucleoside within an RNA enabled that chain to alter substrates other than RNA, that would "extend the enzymatic repertoire [of ribozymes] beyond the currently known examples," he says.