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Pyrrolysine biosynthesized directly Scientists have shown that the 22nd amino acid, pyrrolysine, is biosynthesized as a free amino acid. Like the other 21 genetically encoded amino acids, pyrrolysine (shown) is directly inserted into the growing peptide chain by a dedicated transfer RNA (tRNA). Scientists figured pyrrolysine biosynthesis would parallel biosynthesis of selenocysteine, the 21st amino acid: A lysine would be attached to the pyrrolysine tRNA and then derivatized to give pyrrolysine. But new work by Dieter Söll of Yale University and colleagues suggests otherwise [Proc. Natl. Acad. Sci. USA, published online Aug. 16, http://dx.doi.org/10.1073/pnas.0405362101]. They chemically synthesized a racemic 4-methyl version of pyrrolysine and showed that it (but not lysine) can be attached directly to the pyrrolysine tRNA by an enzyme they dub pyrrolysyl-tRNA synthetase. These results suggest that pyrrolysine is biosynthesized as a free amino acid and then attached to its tRNA. It should therefore be possible to expand an organism's genetic code to include pyrrolysine simply by introducing the genes for pyrrolysine tRNA and its synthetase as well as exogenous pyrrolysine |
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