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Recent excitement in understanding transfer RNA identity.

There was a recent flurry of articles in the press about the "second genetic code." Many scientists wondered 
what this could be. It is an unfortunate term recently coined (1) to describe the sites on each transfer RNA (tRNA) that determine which amino acid will be coupled to it by an aminoacyl tRNA synthetase( AAS). The correct attachment of amino acids to specific tRNA's is crucial to the accurate translation of genetic information from nucleic acid to protein. The "first" genetic code deciphers the rules which govern insertion of specific amino acids in response to the sequence of the messenger RNA. This occurs by alignment of aminoacyl-tRNA's along the mRNA template by base pairing between the anticodon of the tRNA and codons in the template. Because tRNA molecules must interact interchangeably with the protein synthesis apparatus, they all have very similar secondary and tertiary structures, but within this framework variation must exist so that each tRNA is recognizable to its cognate AAS. The term "second genetic code" implies that a common set of rules governs tRNA recognition by AAS's; however, this does not appear to be the case. A better term," tRNA identity" is in common use, which describes the features of a tRNA molecule which make that tRNA recognizable to one AAS and prevent its recognition by all other AAS's.