Three Little Unexpected Children

Abstract

Evolution The genetic code consists of triplets of nucleotides (codons) that are read by complementary triplets (anticodons) in amino acid–carrying tRNAs; for example, the codon CAU is read by an AUG-anticodon in tRNA charged with the amino acid histidine (His). Charting the evolution of the triplets is complicated because almost nothing is known of what came before. The stereochemical hypothesis suggests that codons or anticodons arose through specific recognition of their cognate animo acid. Johnson and Wang looked through the three-dimensional structures of ribosomes from four different species to see if any of the amino acids in the ribosomal proteins might be found in the vicinity of their respective codon or anticodons in the ribosomal RNAs. Although no amino acids were preferentially enriched near their contemporary codons, 11 amino acids were found close to their anticodons. It has been proposed that these 11 amino acids joined the primordial genetic code later on, perhaps during the concurrent evolution of a primitive translation system. Statistical analysis of 4 of the 11 amino acids suggests that they underwent reassignment; His was initially coded by four codons—CAU, CAC, CAA, and CAG—but CAA and CAG were subsequently ceded to the upstart glutamine. Proc. Natl. Acad. Sci. U.S.A. 107 , 10.1073/pnas.1000704107 (2010).