An unusual RNA tertiary interaction has a role for the specific aminoacylation of a transfer RNA.

The nucleotides in a tRNA that specifically interact with the cognate aminoacyl-tRNA synthetase have been found largely located in the helical stems, the anticodon, or the discriminator base, where they vary from one tRNA to another. The conserved and semiconserved nucleotides that are responsible for the tRNA tertiary structure have been shown to have little role in synthetase recognition. Here we report that aminoacylation of Escherichia coli tRNA(Cys) depends on the anticodon, the discriminator base, and a tertiary interaction between the semiconserved nucleotides at positions 15 and 48. While all other tRNAs contain a purine at position 15 and a complementary pyrimidine at position 48 that establish the tertiary interaction known as the Levitt pair, E. coli tRNA(Cys) has guanosine -15 and -48. Replacement of guanosine -15 or -48 with cytidine virtually eliminates aminoacylation. Structural analyses with chemical probes suggest that guanosine -15 and -48 interact through hydrogen bonds between the exocyclic N-2 and ring N-3 to stabilize the joining of the two long helical stems of the tRNA. This tertiary interaction is different from the traditional base pairing scheme in the Levitt pair, where hydrogen bonds would form between N-1 and O-6. Our results provide evidence for a role of RNA tertiary structure in synthetase recognition.