Studies on anticodon-anticodon interactions: hemi-protonation of cytosines induces self-pairing through the GCC anticodon of E. coli tRNA-Gly

The temperature-jump method was used to compare the stability of anticodon-anticodon duplexes formed by the self-association of two tRNAs: yeast tRNA-Asp and Escherichia coli tRNA-Gly. Yeast tRNA-Asp duplexes contain a U/U mismatch while E. coli tRNA-Gly dimers have a C/C mismatch in the middle position of their quasi self-complementary anticodons GUC and GCC, respectively. At neutral pH, it is found that only tRNA-Asp duplexes exist whereas at pH 5.0 only tRNA-Gly duplexes are formed. This reflects the hemi- protonation of the N3 of the cytosines at pH 5.0 which induces a pairing between the two middle residues of the anticodon GCC in E. coli tRNA-Gly. This is the first evidence that a protonated C-C(+) base pair is compatible with the formation of a double helix with antiparallel strands in a natural RNA molecule.