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A MICROBE found in cows鈥� stomachs is forcing biologists to broaden their ideas about the genetic code. Its DNA codes for a previously unseen amino acid, suggesting that the genetic code is richer than anyone realised.

The genetic code consists of sequences of the four DNAbases, A, C, T and G, grouped into threes. Each of these triplets either codes for an amino acid or tells the RNA apparatus that translates the code into protein where to start or stop. Biologists once believed that DNA codes for only 20 different amino acids, but in 1986, they found a 21st.

Now Joe Krzycki and his colleagues at Ohio State University in Columbus have found another. Dubbed pyrrolysine, it is produced by several bacteria-like organisms called archaea, including Methanosarcina barkeri which grows in cows鈥� guts. 鈥淚t鈥檚 like finding a new fundamental particle,鈥� says Michael Chan, one of the Ohio team.

Tests show that the DNA triplet responsible for pyrrolysine is one that usually codes for 鈥渟top鈥�, even in these organisms. However, their cells contain two different types of apparatus for reading DNA, one that reads the triplet as 鈥渟top鈥� and another that interprets it as the code for pyrrolysine.

This ambiguity could be a real danger for an organism if the reading mechanisms got mixed up. 鈥淭here has to be a big pay-off for evolution to muck with the central information process like this,鈥� says Krzycki.

That pay-off is probably the ability to live in dark, murky places: the protein that the bug makes using pyrrolysine helps it to generate energy by metabolising methylamine, a chemical found in swamps and guts.