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."