See
http://nar.oupjournals.org/cgi/content/full/29/20/4125/GKE558F4

This shows that in 10 tRNA Trp acceptor stem sequences
that for these
bacteria, archaea, and eukarya
postion #73 is always a purine (A or G)
and position #1 is always a purine (A or G)

>From a standpoint of preventing dimers
(UV damage on two pyrimidines on the same strand -
C or U)
There may be a reason for these purines.

The Acceptor stem is 

#73 Purine (A or G) / 
#74 pyrimidine C/
#75 pyrimidine C/ 
#76 Purine A

As I suggested before, the two middle C pyrimidines,
may be there to cause a dimer and prevent the stem
from h-bonding to the other strand.
Then the purine A or G in position #73 (and 76)
would be there to prevent the dimer from going 
further on either side.

And as for a purine on #1 or the first base on
the 5' end of the tRNA, it may be there to prevent
a dimer at the top of this stem - thus insuring
a chance that it could h-bond to the 3' end.

Comment?
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