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Referee comments: Referee 2

Posted by PLOS_ONE_Group on 26 Feb 2008 at 12:30 GMT

Referee 2's review:

This is a very interesting manuscript. It is well done and is very original in the conclusion.

The main result is that the two halves of tRNA molecule seem have different evolutionary history and therefore this corroborates the hypothesis of the polyphyletic origin of tRNA genes.

For improving the manuscript, I suggest the following, a few things:
1. On page 3 line 27: introduce the reference: J Theor Biol 1992, 159(2), 199=214;
2. On page 6 line 23: change 'to verify' into 'to corroborate";
3. As in Fig. 1 there are several pairs of amino acids that are in precursor-product relationship (for example, Ser-Cys, Glu-Gln, Phe-Tyr) or in biosynthetic relationship (for example, Ala-Val, Glu-Asp) according to the coevolution theory of the origin of the genetic code (PNAS, 1975, 72(5): 1909=1912), I suggest to introduce a short sentence in the manuscript, on page 9, to stress that point.

After these changes I think that this courageous manuscript might be accepted.

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.