Because of the danger of UV caused dimers
whenever two pyrimidines are adjacent on
an RNA strand, there are only two ways
to code for a safe anticodon (safe being
where there are no 2 pyrimidines side by
side on either the anticodon or the codon)

They are:

purine, pyrimidine, purine

(the 2nd position pyr would pair with
a codon 2nd position pur which some
suggest is connected to hydrophilic 
amino acids).

OR

pyrimidine, purine, pyrimidine

(the 2nd position pur would pair with
a codon 2nd position pyr which some
suggest is connected to hydrophobic
amino acids).

So far so good.  (see other posts
for reasons why this would mostly
be purines matched with philic aa's)

BUT this 3 base anticodon is not
isolated - there are bases on both ends
of the RNA strand that they are on.

Thus if you have
purine, pyr, purine - you have purines
on both ends and no danger of dimers
with their neighbor bases.

BUT, and here is where it gets interesting
for me , if you code 
pyr,pur,pyr there is a real danger
of dimers from the outside pyr
in first or 3rd positions
and a possible pyr next to it
in either the  37 or 33 position.

Such that
33 = pyr
34,35,36 = pyr, pur, pyr anticodon
37 = pyr

thus you have dimer potential in 33 - 34
or 36 -37 position.

Thus it would be highly unlikely for an anticodon
to safely have pyr, pur, pyr coding -

(or for that matter its codon! on the mRNA)

But it seems the bases in tRNA have evolved
to solve this.
In position 37 there is always a purine.
Thus 36-37 can never be two pyrimidines
and there can never be a dimer between
this outside base and position 1 in the anticodon.

On the other end you have 3rd position wobble
at base 34, plus a guaranteed 2 pyrimidines
at position 33 and 32. (Perhaps this weak end
is why there is wobble)

Summary
I think it is more and more clear that dimers
were a major selection force on the genetic code.
There are too many coincidences if it isn't.
And that somehow the mostly purine coding
was associated with hydrophilic amino acids
while only that coding completely free of UV
dimers (whatever environmental conditions that
suggests) coded for hydrophobic amino acids.

Comment?
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 6/18/04 9:49:31 PM