From forensic blood and semen stains in a criminal trial to fictionaldinosaur genes in Jurassic Park, DNA today has become a magnet forpublic attention.
Those dinosaurs roamed, and ruled, the planet a couple of hundredmillion years ago, but the very first life on earth, paleontologistsbelieve, oozed out of the prebiotic soup some 4.5 billion years ago.Their emergence jump-started the since-unbroken chain of replicatingnucleic acids, which eventuated in the DNA and RNA that invest allforms of life on earth today, from retroviruses to homo sapiens.
Which came first at the dawn of life, DNA _ deoxyribonucleic acid_ or RNA _ ribonucleic acid?
Perhaps neither, suggests biochemist David Bartel, a scientist at theWhitehead Institute of Biomedical Research, in Cambridge, Mass.
"I would say," Bartel told BioWorld Today, "that life could havestarted with a different molecule, and a transition from it to a timewhen RNA dominated. It's really very much an open question, ormystery, as to what the molecule actually was."
According to the "RNA World Hypothesis," which many scientistsespouse, he continued, "there was a time early in the evolution of lifein which RNA played a much more important role than it does now,by catalyzing reactions essential to life. Some people say that lifeactually started with RNA."
RNA, of course, is best known today as the molecule that DNA, thestuff that genes are made of, sends out of a cell's nucleus into itsribosomes, where they manufacture proteins. How then, at the pre-protein dawn of life, could the "chicken" of RNA have come beforethe "egg" of DNA?
"No problem," Bartel said, "You can make DNA from RNA byreverse transcription, so there's no reason to think that DNA had tocome first. And RNA could well have taken off by replicating itself."
Operation RNA Bootstrap: Copy Yourself
Many people, he pointed out "think that RNA, or an RNA-likemolecule, could both code for its own replication, and catalyze thelinking-up of small nucleotide units to make copies of itself. Somesort of molecule like that must have been important early in theevolution of life."
For several years, Bartel has been pursuing this possibility of what hecalls "in vitro evolution," with the help of DNA synthesizers andcombinatorial chemistry. He sees its utility as being in the realm ofthe intellect rather than the marketplace.
"Studying the ability of RNA to use information from RNA to makemore RNA is primarily an intellectual question," he said. "There areeasier ways to make RNA, now that we have proteins."
He's not alone in this pursuit.
"There are people working in many different areas," Bartel pointedout, "on what the geological conditions must have been like when lifestarted. What kinds of molecules could have arisen by prebioticsynthesis?"
Today's issue of Nature carries his latest paper on the subject, titled:"RNA-catalyzed RNA polymerization using nucleosidetriphosphates."
By definition, the ongoing global project to map the entire humangenome began with starting material taken from the cells of livingpeople. By contrast, Bartel's ongoing effort to recreate primordialRNA got his openers by DNA synthesis.
"We started with a catalytic RNA that did not come from nature," hesaid. "We derived it by starting with 1015 [one quadrillion] randomsequences, and selecting those that could catalyze an RNA ligationreaction. Those sequences came from DNA combinatorial synthesis,and then transcribed into RNA."
He explained: "That synthesis takes a DNA synthesizer and puts inone pot four reagents, each containing one of the four DNA bases,namely, A [adenosine], C [cytosine], G [guanosine], and T[thymidine]. Then," he continued, "as each oligonucleotide is made,there's an equal chance of each of the four bases adding on. So by thetime you've made something that's any length at all, they're all goingto be different and random."
The next step was to "transcribe that DNA into RNA molecules thathad no relevance to any known or unknown RNA, or any RNA innature. Now we had one quadrillion different RNAs."
100 Sequences In A Quadrillion, And Still Counting
So he and his co-author searched among them for RNAs that couldmake themselves bigger by carrying out an RNA ligation.
"We got over 100 different incredibly rare sequences that couldcatalyze that reaction. But since we started with 1015, that still wasn'tvery many of the total."
After "some evolution by making point mutations in those sequences,we started selecting those that could do the ligation reaction faster.Then we looked for individual sequences that could add just one baseat a time to the RNA chain, like one step of RNA polymerization."
The team was in effect, trying to emulate the RNA enzyme, Bartelsaid, "which isn't found in nature today, but may have been 4.5billion years ago."
Eventually, they succeeded in getting their primitive RNA enzyme toadd first three, then six nucleotides to the growing chain.
"That's not nearly long enough," Bartel pointed out, "for that RNA tobe able to replicate itself, but it is a better indication that it could.That is, if it can do six, maybe another version of it can do more.Ultimately, there may be an RNA that can make a complete copy ofitself.
"That would be the proof of principle," he concluded. "Right now itwould require about 100 nucleotides. So we're a long way from that,but we're also significantly further than we used to be."
Bartel began his project in 1993 as a doctoral fellow in the laboratoryof molecular biologist Jack Szostak a professor of molecular geneticsat Harvard Medical School.
"His work is really a breakthrough," Szostak told BioWorld Today,"on the way to trying to make or evolve an RNA molecule that is anRNA polymerase, good enough to copy itself. And of course it's veryexciting in terms of thinking about the origin of life, or very primitivekinds of cells." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.