Agmatidine (2-agmatinylcytidine, symbol C+ or agm2C) is a modified cytidine present in the wobble position of the anticodon of several archaeal AUA decoding tRNAs. Agmatidine is essential for correct decoding of the AUA codon in many archaea and is required for aminoacylation of tRNAIle2 with isoleucine. Agmatidine - wiki
Two related papers -
[The structural basis for specific decoding of AUA by isoleucine tRNA on the ribosome
The role of the agmatidine and lysidine modifications in isoleucine decoding is a notable example of the essential function of modifications in tRNA and, more generally, in RNA biology. It is increasingly evident that although the genetic code was first elucidated over 40 years ago, we are just beginning to appreciate the sheer complexity of ensuring accuracy in protein synthesis on the ribosome.