View Full Version : Amino acid no. 22 found!

05-31-2002, 05:14 PM


(This story embargoed until 2 PM ET, Thursday, May 23, 2002 to coincide with
publication in the journal Science.)

COLUMBUS, Ohio - Two teams of researchers from Ohio State University reported
today that they had identified the 22nd genetically encoded amino acid, a
discovery that is the biological equivalent of physicists finding a new
fundamental particle or chemists discovering a new element.

Two papers describing the discovery appear in the current issue of the journal
Science. Prior to this, scientists had believed that there were only 21 natural
amino acids -- the key building blocks of proteins.


"I think this work will cause researchers to start looking at genetic sequences
that they might have thought at first were simply aberrations."

For 30 years after the discovery of the structure of DNA and the unraveling of
the genetic code, scientists believed that there were only 20 natural amino
acids. Then in 1986, researchers broke that numerical barrier announcing that
the 21st had been discovered.

Finding a 22nd suggests that even more of these basic biological building
blocks may be found using modern genome sequencing techniques.

The discovery grew out of some very basic biochemistry examining how a
particular type of microbe - methanogens - can convert methyl-containing
compounds into methane. While researchers have long understood the biochemical
mechanisms for how acetate and carbon dioxide are converted to methane, they
didn't understand how a common class of compounds - the methylamines - are
transformed into this gas.

One research group, led by Joseph A. Krzycki, an associate professor of
microbiology, had been working for several years with a particular strain of
microbe, Methanosarcina barkeri. This organism, a member of the recently
identified domain Archaea, is able to convert monomethylamine, dimethylamine
and trimethylamine into this greenhouse gas.

Krzycki's research group had isolated specific proteins related to the process
in 1995 and, two years later, they had isolated and sequenced one of the genes
responsible. Then in 1998, they published a paper showing that the gene had a
component called an in-frame amber codon that behaved unusually.

Codons are three-letter "words" identifying the bases DNA uses to specify
particular amino acids as building blocks of proteins. Normally, codons signal
the start of a protein, its end or a particular amino acid used to construct
it. Surprisingly, the codon Krzycki's team identified should have signaled a
stop to protein building but it did not.

"Joe and his colleagues found this happening in genes important for all three
of the methylamine compounds - something that wasn't supposed to happen,"
explained Michael Chan, an associate professor of biochemistry and chemistry at
Ohio State. Chan led the second research team that identified and determined
the structure of the amino acid.

The realization of the codon's odd behavior suggested the possibility of a new
amino acid, but the researchers knew there might be other explanations as well.
Krzycki and his colleagues sliced the protein into smaller bits called
peptides, and began sequencing them, a process which usually ultimately reveal
the amino acid responsible for the protein.

"That all seemed to point to this being just lysine, one of the normal amino
acids," Chan said. Regardless, Krzycki asked Chan and Ph.D. student Bing Hao to
start working on deducing the crystalline structure of the protein containing
the amino acid. At the end of the two-year process, Hao and Chan had determined
the structure of the protein, part of which revealed a new amino acid.

At the same time, Krzycki was looking for other evidence. He, along with
doctoral students Gayathri Srinivasan and Carey James, was eventually able to
identify the specific transfer-RNA (tRNA) needed to insert the new amino acid
into protein, as well as another important enzyme essential to the process.
These two discoveries, along with the detailed crystalline structure, convinced
the teams that they had found a new genetically encoded amino acid --
pyrrolysine - the 22nd known to science.

"We realized that we had to know which tRNA would decode that amber codon,"
Krzycki said. "Finding it was an essential part of the puzzle."

He believes this will be a very rare amino acid, given the fact that it has
taken so long to identify it. However, Krzycki believes it is likely to be
found in other situations - in other organisms - aside from methanogens. He's
philosophic about the importance of the discovery: "This shows us that the
genetic code, and therefore, evolution is much more plastic than people might
have thought."

Chan agrees, pointing to the strong possibility that finding a 22nd genetically
encoded amino acid should stimulate the search for a 23rd or a 24th. "With so
many researchers dissecting so many genomes now, it's reasonable to suggest
that there might be more waiting to be found.

"I think this work will cause researchers to start looking at genetic sequences
that they might have thought at first were simply aberrations," he
said. "Instead, they might signal discoveries like ours."

The research was supported by the National Science Foundation, the National
Institutes of Health, the Department of Energy and the Alfred P. Sloan
Foundation. Along with Krzycki, Chan and Hao, Weimin Gong and Tsuneo Ferguson
worked on the project.

Short N Buff
06-01-2002, 12:47 PM
i'm confused. (i took ap bio two years ago) a 3 set codon makes 1 amino acid correct? but i remmber looking at a chart w/ all the combonations and their coresponding amino acid. where did the 21 and 22nd one come from?