The importance of being modified: tautomeric forms of pyrimidines provide expanded use of the genetic code

Tautomeric shifts of five-modified uridines have been established in, Escherichia coli tRNAVal cmo5UAC, as well as in the human cytoplasmic tRNALys3 mcm5s2UUU. These shifts enable the recognition of multiple codons with the wobble base pair in Watson–Crick geometry, suggesting an energetic preference for perhaps an enhanced efficiency with this conformation. Thus, modifications at the fifth-position of wobble position uridines appear to be a common mechanism for expanding tRNA’s codon recognition. One deviation from the universal code which is particularly interesting is the reading of AUG and the universal isoleucine codon AUA as methionine in both the A- and P-sites of the mitochondrial ribosome of most metazoans. We have discovered a mechanism by which five-formylcytidine (f5C) at the wobble position of human mitochondrial tRNAMet (hmtRNAMet f5CAU) expands the decoding ability of tRNA, enabling it to read AUA as methionine. We have also found that another modified cytidine can restrict wobble-base formation. This underscores the ability of cytidines, as well as uridine, modifications within the anticodon loop to modulate the decoding ability of the tRNA, providing an insight into decoding mechanisms and evolution of the genetic code.