New letters for life's alphabet

摘要

All known life forms store and transmit information from generation to generation using the bases found in nucleic acids: adenine, cytosine, guanine, thymine and uracil. In nucleic-acid double helices, these form base pairs (guanine with cytosine, and either adenine with thymine in DNA, or adenine with uracil in RNA), which are mostly orthogonal - that is, little pairing occurs between other combinations of bases. However, this alphabet' seems to be an accident of history rather than a functional necessity, given that other orthogonal base pairs have been synthesized and shown to be processed by DNA-replication enzymes in vitro. Because life on Earth is biochemically uniform, the formal possibility of alternative alphabets requires strong experimental proof. In this issue, Malyshev et al. (page 385) provide just such a proof, by conclusively showing that an unnatural base pair can be stably propagated in the bacterium Escherichia coli.