Part of the maturation of tRNA is a step that
involves RNase D that removes two nucleotides
from the stem. After that we are left with an
acceptor stem that has bases CCA. The two nucleotides
that are cleaved (E. coli tRNA Tyr as the example) 
are UC - two pyrimidines. These could form a 
pyrimidine dimer if struck with UV.

This suggests that there are two possible dimers
(pyrimidines side by side that form dimers when
struck by UV) with one on each side of the A on
the acceptor stem.

Therefore we have CC-A/UC
where CCA is the stem, and UC the 2 bases cut off
by the RNase D
CC and UC are both pyrimidine pairs adjacent on the
same strand. Thus both are liable to dimers from UV.

It seems to me that dimer damage could play a part
in coding a tRNA where it would be advantageous not
to have base pairing as in this stem end. 
(Also note the pyrimidine pair
before the anticodon at the other end of the tRNA).

Other clues to consider (though I don't
exactly know what they suggest)

Note that most mRNA's have purine at the 5' end.

Note that  RNase P (RNA + protein but note
"under nonphysiological conditions the RNA molecule
alone can catalyze accurate cleavage) can be considered
a ribozyme. It's job is to cleave the 5' end of the
tRNA.

Note that a single RNA polymerase synthesiszes all
prokaryotic RNA's: mRNA, rRNA, and tRNA.
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