A tiny methane-producing microbe has revealed the existence of a new amino acid, researchers report. Two studies published in the current issue of the journal Science describe pyrrolysine, a nonstandard amino acid encoded in the RNA of bacteria-like members of the group Archaea. The findings suggest that the genetic code may be more adaptable than previously believed.

When scientists first began to unravel the genetic code more than four decades ago, they determined that the four DNA bases formed 64 unique groups of three called codons. Further investigation revealed that 20 of these triplets identified amino acids, the building blocks of proteins, and the remainder contained instructions for when to start and stop protein production. Scientists identified selenocysteine, the 21st amino acid, in 1986. In the new work, Joseph A. Krzycki and his colleagues at Ohio State University were investigating a so-called amber codon in Methanosarcina barkeri, organisms that produce natural gas. The researchers found that the codon, which should have signaled a halt to protein production, instead acted as the blueprint for a previously unknown amino acid, pyrrolysine. The researchers also uncovered the specialized transfer-RNA necessary to insert pyrrolysine into proteins. In the second Science paper, biochemist Michael Chan of Ohio State and his colleagues detail the structure of the 22nd amino acid.

Considering how long it took scientists to identify it, pyrrolysine is most likely a very rare amino acid, according to Krzycki. But Chan is optimistic that more such discoveries lie ahead. "With so many researchers dissecting so many genomes now," he says, "it's reasonable to suggest that there might be more waiting to be found."

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