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FROM amoebas to people, living species make proteins from the same repertoire
of 20 amino acids. But now researchers in Japan have persuaded the
protein-building machinery of an E. coli bug to include two amino acids
that don鈥檛 occur naturally.

Masahiko Sisido and his colleagues at Okayama University achieved this feat
by artificially expanding the genetic code. Their technique could be used to
study how proteins change shape as they carry out their tasks, or even to create
new proteins that trap sunlight and convert it into a usable fuel.

When cells make proteins, they first copy genes to make molecules called
messenger RNA (mRNA). The sequence of bases in these molecules tells the cell
how to string amino acids together to make proteins. Normally, each set of three
bases represents an amino acid, and these sets, or codons, are recognised by
other molecules called transfer RNA (tRNA). There are different tRNA molecules,
carrying different amino acids, for each codon.

To add new amino acids to a protein called streptavidin, Sisido鈥檚 team
attached them to a rare type of tRNA molecule that reads a set of four bases as
if it were a codon of three bases. Then they tinkered with the mRNA for the
protein so that its sequence included codons of four bases that would be
recognised by the tRNA molecules. In this way, the artificial amino acids were
slotted into the protein as it was made.

Researchers have managed to make proteins containing artificial amino acids
before. However, only one artificial amino acid could be added to each protein,
whereas the new technique can add many. This should help biologists work out the
shape changes that allow proteins to do everything from triggering nerve
impulses to digesting your lunch. Researchers could watch those changes as they
happen by adding two amino acids containing fluorescent molecules. The
fluorescent spectra emitted would depend on how far they are from each other,
which in turn depends on the protein鈥檚 shape.

Sisido speculates that proteins containing artificial amino acids will be
useful in industry. For instance, by incorporating amino acids that absorb solar
energy and eject electrons, it could be possible to make solar-powered enzymes
that make fuel or digest waste. 鈥淲e will be able to extend the biological
functions of proteins,鈥� he predicts.

鈥淚t鈥檚 a very clever technique, with lots of potential,鈥� agrees David Millar,
a biophysical chemist at the Scripps Research Institute in La Jolla, California.
He thinks the technique could be useful for studying how strings of amino acids
fold into proteins. 鈥淓ventually, you could learn how to predict a protein鈥檚
three-dimensional structure from its amino acid sequence, which would help
decipher the information pouring out of the human genome project,鈥� he says.
Protein folding is also important in understanding diseases such as Parkinson鈥檚,
in which proteins in the brain are wrongly folded.