Amazing. Remarkable. Striking. Truly startling.
All four of these words appear on a one-page editorial in the Sept. 27,1996, Science. Its title: "A parallel spliceosome." Its author:molecular biologist Timothy Nilsen, who directs the center for RNAbiology at Case Western Reserve School of Medicine in Cleveland.
Spliceosomes are the molecular railroad-yard crews that uncoupleand side track empty freight cars, before reassembling an outboundtrain of messenger RNA for its trip through the protein-buildingribosomes.
Those empties are the introns, strings of apparently useless DNA,(and its counterpart RNA), in a full-length gene.
After introns _ "intervening sequences" _ were discovered in thelate 1970s, researchers identified spliceosomes as the hugemacromolecular complexes that tease these stretches of silent DNAloose from the exons, or coding sequences. They then splice theexons together to form functional, protein-expressing messengerRNA.
"A spliceosome," Nilsen told BioWorld Today, "is a very largestructure, as big as a ribosome. It's composed of three RNAs andabout 60 proteins."
The reason for Nilsen's amazement is the discovery reported in theScience paper on which his editorial commented. That article's titleonly hints at its novelty: "Highly diverged U4 and U6 small nuclearRNAs required for splicing rare AT-AC introns."
That paper's author, molecular biologist Joan Steitz of YaleUniversity's Howard Hughes Institute, explained the excitement toBioWorld Today: "For the past 15 years, a number of laboratories allover the world _ including mine _ have been working on thespliceosome's machinery. We now know that it's composed of smallnuclear ribonuclear proteins.
"So the presumption was," Steitz continued, "that this one machinery,which has been identified in all eukaryotic organisms from yeast toman, basically works on all introns in pre-messenger RNA to cutthem out."
Then comes the kicker: "The important finding in our Sciencearticle," she said, "is that hiding in our cells all this time is in fact acompletely different machinery, which does the same thing for someof our introns at about one one-hundredth the level of the knownspliceosome's machinery.
"Where does this new spliceosome come from?" Steitz askedrhetorically. "Why do we have a completely different system whenwe've been studying a system that we know is versatile, that canexcise all sorts of introns, with different lengths, different sequences,can be regulated to give alternative splicing, and seems to be God'sgift to be able to act on anything?
"Now all of a sudden we discover there's another spliceosome outthere, which acts on the same pre-messenger RNA."
At this year's Miami Winter Symposium, where she won theDistinguished Service Award, Steitz spoke on: "The cell nucleolus:Yet another RNA machine?" (See BioWorld Today, Feb. 13, 1996,p. 3.)
There are about ten introns in the average human gene, ranging inlength from 70 bases to thousands. Their purpose is murky at best."We don't know why there are introns," Steitz observed.
"There's a lot of speculation as to their function," Nilsen said,adding, "It's quite clear that intervening sequences allow for theproduction of multiple proteins from a single gene, by alternativesplicing." He cited by way of example, alpha-tropomyosin, a muscle-fiber component, of which 64 proteins can be generated from thesame gene.
A Surprise Lurking Deep Among Gene Sequences
"These types of pre-RNA introns," Nilsen added, "are not reallyimplicated in any disease processes. Mutations in splice sites areinvolved in some diseases, such as thalassemia, and many diseasesare caused by mis-splicing of pre-RNA, but those are not reallytherapeutic targets, that I'm aware of."
Nilsen is a scientific advisor to Innovir Laboratories Inc. in NewYork. He sees no "specific biomedical or biotechnologicalapplication for [Steitz's] findings. It's just exciting from anintellectual point of view _ something that was totally unexpected."
Steitz found the second spliceosome lurking under deep cover amonggene sequences in the data base. "Introns have the sequences at theirends," she explained, "that are recognized by the splicing machinery,so should be cut out and spliced back together. And what peoplestarted noticing a couple of years ago was that one intron in 500would have different sequences at its ends, that didn't fit theconsensus."
What she and others then realized was that "perhaps these smallnuclear ribonuclear proteins that are part of the spliceosome in facthad sequences that would match those non-canonical splice-sitesequences."
From that point of departure, she and her post-doc, Woan-Yuh Tarn,the paper's first author, "identified all of those components in thisnew spliceosome."
As for the finding's practical utility, Steitz's message is: "Whateverybody who's looking at genes and genomes, and using thatinformation, needs to know, is that when you're scanning for intronsand exons, don't just look for the one type of sequence at thejunctions. There's another sequence, or pair of sequences, that you'vegot to be looking for, or you're going to miss this whole class."
A spliceosome contains at least 150 proteins, "so there are at least150 genes required to make one," Steitz pointed out. "Each of thesegenes contains its own suite of introns, which are stripped out by theirown complement of spliceosomes. The chicken-and-egg question is:Where do the spliceosome's spliceosomes come from?"
Her answer: "That's life." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.