**Differential interference contrast micrograph of __E. crassus__.**
Gladyshev and his colleagues analyzed the __E. crassus__ genome sequence and found eight genes for proteins that incorporate selenocysteine, four of which contained multiple UGA codons. By expressing these genes in human embryonic kidney cells and using mass spectrometry on the __E. crassus__ proteins, Gladyshev's team showed that UGA differentially coded for either cysteine or selenocysteine. They also found that which amino acid was incorporated into the protein depended on the proximity of the UGA codon to a specific structure in the 3' untranslated region called the "Sec insertion sequence," or SECIS. linkurl:John Atkins,;http://www.bioscience.utah.edu/mb/mbFaculty/atkins/atkins.html a molecular biologist at the University of Utah in Salt Lake City and University College Cork in Ireland, who was not involved in the study, said that dual coding might be a "specialist situation," rather than a general phenomenon. Still, the observation that the SECIS element was differentially affecting the ribosome at various UGA triplets has major mechanistic implications about the fine-tuned control of ribosome-based protein synthesis, he said. "It shows that there must be very specialized folding of RNA in the 3' untranslated region to talk to the ribosome," he told __The Scientist__. "The redefinition [caused by SECIS] is not of the ribosome as a whole; rather, the signal is determining a localized definition." But the paper's findings might not be all that novel, cautioned Yale University microbiologist Dieter Söll. He noted that in some __Candida__ species, the CUG codon is translated as both leucine and serine, even in the same gene, albeit by a single ambiguous tRNA rather than two separate tRNAs as Gladyshev's team found in __E. crassus__. "This already showed that you can have the same codon in one gene [encode] two amino acids," Söll told __The Scientist__. "Really, it simply shows that nature has more than one way of doing the same thing." Gladyshev disagrees. "It's really a completely different situation," he said. In __Candida__, the tRNA doesn't discriminate between amino acids, he noted. Rather, it randomly inserts either serine or leucine, "whereas in our case there is a specific insertion of one amino acid or the other, depending on the presence and availability of the RNA element." Söll also noticed a technical error in the paper. In Figure 2, the putative cysteine tRNA that Gladyshev identified from the genome sequence had a cytosine residue at a terminal "charging site." But all other known cysteine tRNAs have uracil instead, Söll noted. Gladyshev said the figure contained a mistake: the true nucleotide was, indeed, a uracil. "I would like to thank... the expert who noticed this," he wrote in an email. The sequence he submitted to Genbank was correct, he noted, and he has contacted __Science__'s editors to issue a correction. Gladyshev is now searching for other organisms that display dual coding. He is also investigating mammalian selenocysteine-containing genes to investigate whether the position of the SECIS element affects the incorporation of selenocysteine at different codon sites. __Image courtesy of Lawrence Klobutcher
**Related stories:__***linkurl:Nirenberg's genetic code chart, 1961-66;http://www.the-scientist.com/2007/6/1/88/1/
[June 2007]*linkurl:Extending the genetic code;http://www.the-scientist.com/article/display/21537/
[15th August 2003]*linkurl:And then there were 22;http://www.the-scientist.com/article/display/20435/
[7th June 2002]
