Double duty. This protozoan encodes two amino acids with one codon.
Credit: Lawrence Klobutcher
Posted on 01/09/2009 1:30:55 AM PST by neverdem
Double duty. This protozoan encodes two amino acids with one codon.
Credit: Lawrence Klobutcher
Call it the genetic version of a double-entendre. Scientific dogma dictates that various three-letter combinations of our genetic sequence each "mean" exactly one thing--each codes for a particular amino acid, the building block of proteins. But a protozoan named Euplotes crassus appears to be more versatile: One of its three-letter combinations has two meanings, coding for two different amino acids. Although the find may seem trivial, it poses a major challenge to more than 4 decades of scientific thinking.
It's a long road from gene to protein. First, enzymes zip along DNA, producing ticker-tape-like messages called messenger RNA. Bloblike molecules called ribosomes then glom onto this RNA, translating its three-letter codes--known as codons--into amino acids, such as glycine and tryptophan. Once the amino acid chain is complete, the cell modifies it further until it's presentable as a bona fide protein. Although scientists are continually refining their understanding of this process, one thing seemed constant: Codons coded for one--and only one--amino acid.
E. crassus apparently didn't get the memo. Researchers led by biochemist Vadim Gladyshev of the University of Nebraska, Lincoln, have discovered that this protozoan can encode two amino acids with one codon. The codon, UGA, codes for an amino acid known as cysteine in E. crassus. But the researchers found that the protozoan also uses UGA to make the amino acid selenocysteine, a dual amino acid coding not yet seen in any other organism. "We were very surprised," says Gladyshev, whose team reports its findings tomorrow in Science.
The trick seems to be a genetic element known as the selenocysteine insertion sequence (SECIS) located at the end of the messenger RNA strip. The element produces a physical loop in the RNA, Gladyshev explains, which interacts with ribosomes and changes UGA's message. Without it, the codon codes for cysteine; with it, the codon codes for selenocysteine.
Next, the team aims to understand exactly how the SECIS element manipulates the UGA codon and if this codon versatility occurs in other organisms. "There are probably variations on this theme out there," says biochemist Jamie Cate of the University of California, Berkeley. At the very least, the find may necessitate a rewrite of science textbooks. Says Cate, "It's sort of like a warning shot not to get too comfortable with what we think is going on."
Genetic Code Supports Targeted Insertion of Two Amino Acids by One Codon
Science 323 (5911), 259. [DOI: 10.1126/science.1164748]
| PDF »
| Supporting Online Material »
“Importance of Selenocysteine for Health
It has been discovered that HIV-1 encodes a
functional selenoprotein. Patients with HIV have
been shown to have a lower than average blood
plasma selenium level. These individuals are also
more succeptible to certain symptoms such as
cardiac myopathy. They are also found to contain
several low molecular mass selenium compounds.
These compounds are thought to be a 7-9 kD
selenoprotein which is encoded by the HIV genome.
This protein is in fact a glutathione peroxidase.
Patients with HIV often exhibit low selenoprotein
concentrations in their cells. Thus AIDS patients
are reccommended to take selenium supplements to
allow their cells to continue to produce these
valuable redox proteins”
Ya wants more yer on yer own, I don’t work fer free.
Did any one ask you?
Piece of cake ping.
Now if the Globull Warming nazis can only get this thru their semipermeable membranes (aka, SKULLS)....
A couple of nitpicks. Cells dont use codons to “make” an amino acid, it uses them to know which amino acid to insert into the protein chain. The second nit is that selenocysteine is not one of the typical amino acids, it is very closely related structurally and functionally to cysteine. The 2 are probably interchangable in a protein. Lets see a cell use the same codon for very different amino acids, like glycine and tryptophan. It wouldnt last long.
In most organisms, UGA doesn’t encode for any amino acid at all. It is one of the “stop” codons that actually ends protein synthesis. Interesting that this organism would use the codon for two when most don’t use it all.
I guess that means the check is not in the mail. o.k....
I was watching the earth being destroyed by asteroids when I posted that from, “Selenocysteine:
The 21st Amino AcidBy:
M. Gill, S. Gupta, L. Zichittella” so any lack is my fault.
Besides, who wants nits? Pick away!
This from the posted article,
“Researchers led by biochemist Vadim Gladyshev of the University of Nebraska, Lincoln, have discovered that this protozoan can encode two amino acids with one codon. The codon, UGA, codes for an amino acid known as cysteine in E. crassus. But the researchers found that the protozoan also uses UGA to make the amino acid selenocysteine, a dual amino acid coding not yet seen in any other organism. “We were very surprised,” says Gladyshev, whose team reports its findings tomorrow in Science.”
Are their findings new? I found this in PubMed from over ten years ago:
“SELENOCYSTEINE Annu Rev Biochem. 1996;65:83-100
Stadtman TC.
National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Selenocysteine is recognized as the 21st amino acid in ribosome-mediated protein synthesis and its specific incorporation is directed by the UGA codon. Unique tRNAs that have complementary UCA anticodons are aminoacylated with serine, the seryl-tRNA is converted to selenocysteyl-tRNA and the latter binds specifically to a special elongation factor and is delivered to the ribosome. Recognition elements within the mRNAs are essential for translation of UGA as selenocysteine. A reactive oxygen-labile compound, selenophosphate, is the selenium donor required for synthesis of selenocysteyl-tRNA. Selenophosphate synthetase, which forms selenophosphate from selenide and ATP, is found in various prokaryotes, eukaryotes, and archaebacteria. The distribution and properties of selenocysteine-containing enzymes and proteins that have been discovered to date are discussed. Artificial selenoenzymes such as selenosubtilisin have been produced by chemical modification. Genetic engineering techniques also have been used to replace cysteine residues in proteins with selenocysteine. The mechanistic roles of selenocysteine residues in the glutathione peroxidase family of enzymes, the 5’ deiodinases, formate dehydrogenases, glycine reductase, and a few hydrogenases are discussed. In some cases a marked decrease in catalytic activity of an enzyme is observed when a selenocysteine residue is replaced with cysteine. This substitution caused complete loss of glycine reductase selenoprotein A activity.
PMID: 8811175 [PubMed - indexed for MEDLINE]”
What have the researchers in “Science” found that is new?
Now that the earth survived the asteroids, what am I missing here?
“What have the researchers in “Science” found that is new?”
I don’t have time to read the articles, but I can tell you that it is common for scientists not to be familiar with older literature. This may be a case of rediscovering that which is already known. Irt has happened to me where people from reputable labs (who should know better) have published results similar to those I had published 5 years prior. I would be suprised if something like that got past the reviewers in Science.
Maybe a confirmation of what was suspected? I’ll go back and reread the articles. I can be a bit slow at times.
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