How two amino acids become one

摘要

The first genetically encoded amino acid was identified more than two centuries ago, but new ones are still being found. The most recently reported one - the twenty-second - is pyrrolysine (Pyl), which was found1'2 in 2002 at the active sites of methyl-transferase enzymes obtained from a methane-producing archaeon. Like the 20 common amino acids that are incorporated into cellular proteins, Pyl is synthesized in the cytoplasm and incorporated at a specific position in a growing polypeptide chain during translation3. However, it was the only genetically encoded amino acid for which a biosynthetic pathway had not been established. On page 647 of this issue, Krzycki and colleagues4 report that the essential amino acid lysine is the sole precursor of Pyl, and they define the enzymatic steps for the conversion of two L-lysine molecules into one molecule of L-Pyl.