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Abstract

The underlying structure of the canonical amino acid substitution matrix (aaSM) is examined by considering stepwise improvements in the differential recognition of amino acids according to their chemical properties during the branching history of the two aminoacyl-tRNA synthetase (aaRS) superfamilies. The evolutionary expansion of the genetic code is described by a simple parameterization of the aaSM, in which (i) the number of distinguishable amino acid types, (ii) the matrix dimension and (iii) the number of parameters, each increases by one for each bifurcation in an aaRS phylogeny. Parameterized matrices corresponding to trees in which the size of an amino acid sidechain is the only discernible property behind its categorization as a substrate, exclusively for a Class I or II aaRS, provide a significantly better fit to empirically determined aaSM than trees with random bifurcation patterns. A second split between polar and nonpolar amino acids in each Class effects a vastly greater further improvement. The earliest Class-separated epochs in the phylogenies of the aaRS reflect these enzymes’ capability to distinguish tRNAs through the recognition of acceptor stem identity elements via the minor (Class I) and major (Class II) helical grooves, which is how the ancient operational code functioned. The advent of tRNA recognition using the anticodon loop supports the evolution of the optimal map of amino acid chemistry found in the later genetic code, an essentially digital categorization, in which polarity is the major functional property, compensating for the unrefined, haphazard differentiation of amino acids achieved by the operational code.

Item Type:	Article
Authors/Creators:	Shore, JA and Holland, BR and Sumner, JG and Nieselt, K and Wills, PR
Keywords:	amino acid substitution matrix, aminoacyl-tRNA synthetase, aaRS phylogeny, code expansion, parameterized matrices, size, polarity
Journal or Publication Title:	Journal of Molecular Evolution
Publisher:	Springer-Verlag
ISSN:	0022-2844
DOI / ID Number:	10.1007/s00239-019-09918-z
Copyright Information:	Copyright 2019 Springer Science+Business Media, LLC, part of Springer Nature
Related URLs:	Google Scholar: Holland, BR UTAS Profile: Holland, BR Google Scholar: Sumner, JG UTAS Profile: Sumner, JG
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