New Amino Acid Discovered; Fundamental Building Block of Life
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.
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 re”>chers 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.
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Contact: Joseph A. Krzycki, (614) 292-1578; Krzycki.1@osu.edu
or Michael Chan, (614) 292- 8375; Chan.136@osu.edu.
Written by Earle Holland, (614) 292-8384; Holland.8@osu.edu.
