It's been a while since I was active in the ribosome field
but as I remember it the CCA is added post-transcriptionally
to all tRNAs. Moazed and Noller (Cell 57, 585-97, 1989)
showed that the terminal CCA interacts directly with certain
residues in the peptidyl transferase region of the large
subunit rRNA and that these residues are universally conserved.
The potential for base-pairing between CCA and the LSU rRNA
residues exists in both the A and P sites. Therefore, the
sequence at the 3' end of tRNAs is important to accurately
position the tRNA during peptidyl transfer and the sequence
is conserved due to the need to interact with complementary
bases in more than one region of the ribosome.
Hope this helps (and that it is accurate!)
C. Graham Clark, Ph.D.
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892
e-mail: cge at cu.nih.gov