Boreskov

A UNIVERSAL TREND OF NUCLEOTIDE ASSYMETRY IN TRNAS SUGGESTSA COMMON THERMOPHILIC ORIGINTitov I.I.Institute of Cytology and Genetics, SB RAS, Novosibirsk, RussiaNovosibirsk State University, Novosibirsk, Russiae‐mail: titov@bionet.nsc.ruIntroduction. The origins of the Last Universal Common Ancestor (LUCA) of life beingshave been covered by a secret veil. Most studies indicate that LUCA was a (hyper)thermofile.Transfer RNAs (tRNAs) are the ancient nucleic acid molecules that can be found in alltaxa. They charge specific amino acids, bind to 16S rRNA and recognize mRNA codons,establishing a genetic code that translates mRNA into protein. Since tRNAs are central totranslation their sequences are fundamentally delimited by their structure and function.Slowly varying tRNA sequences can carry deep evolutionary signatures that were left behindwhen the living world diversified.tRNA nucleotide substitutions have been extensively studied earlier to uncover thestructural make‐up of the molecule which is critical to perform multiple tRNA functions. Inthis paper I used well‐established phylogenetic methods to detect tRNA single substitutionsfor each of 20 iso‐acceptor tRNA families in 123 taxa and found that tRNAs slowlyaccumulate adenine and uracil and correspondingly lose guanine and cytosine nucleotides. Asimilar work [1] has compared sets of orthologous proteins and revealed a universal trend ofamino acid gain and loss but the underlining mechanism is still under debate [2, 3].Methods and Algorithms: tRNA aligned sequences were drawn from http://www.unibayreuth.de/departments/biochemie/trna/.From them 20x123=2460 tRNAs were extracted,each corresponding one amino acid of one taxon of 123 taxa representing all three domainsof life ‐ Bacteria, Archaea and Eukaryota.To reveal favored nucleotide substitutions I calculated fluxes for each of 20 tRNA familiesas well as Jordan and colleagues have analyzed amino‐acid fluxes in proteins [1] (Fig. 1).Common ancestorS1 S2 SFigure 1. A maximum parsimony evaluation of simple 3‐taxon trees.All 123 taxa were splitted into closest triples, in each of them S1 andS2 were two closer taxa and S was more divergent taxon. Thenucleotides shared by S1 and S2 were assumed to exist in the commonancestor and if nucleotide in S was different a single substitutionoccurred along the branch leading to S. Positions differing in all 3 taxawere deemed non‐informative and were excluded from consideration.By these means the transition matrix was calculated: its asymmetrydefines the directions of substitution rates.116