Two research teams at Ohio State University have discovered pyrrolysine, the 22nd genetically encoded amino acid and the first to be found since selenocysteine in 1986 (Science 2002, 296:1459-1462; Science 2002, 296:1462-1466).

Joseph A. Krzycki and colleagues came across the residue while sequencing a gene from Methanosarcina barkeri, a methanogenic anaerobe found in such places as swamps, rice paddies and oceanic sediments. Meddlesome UAG stop codons kept turning up in the sequence of a gene encoding methylamine methlytransferase, a protein that enables methane formation in the bacterium. "We basically thought it was a sequencing error at first," Krzycki told The Scientist. "Then we were doing whole genome PCR and recognizing that this just didn't go away. We went to the RNA and found even then that this message was present. That convinced us this was a real phenomenon."

Chemical analyses suggested that the troublesome stop...

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