The structural geometry of co-ordinated base changes in transfer RNA

The tertiary structure of the central region of yeast tRNA^Phe is maintained by a trans-bonded G · C pair and by three hydrogen-bonding systems each involving three bases (“triples”). All other tRNA sequences which have four base pairs in the D stem and five nucleotides in the extra loop can conserve the triple base bonding arrangements and the trans-bonding of residues 15–48 so that the long helix formed by the TΨC and amino acid stems can be tied to the augmented D helix in the same way. The co-ordinated base changes and the alternative hydrogen-bonding schemes which make this possible are described in detail.