Boreskov

Results: Considering the changes between the ancestor and the descendant taxa atbranching nodes 1972 non‐informative positions and 24415 single substitutions have beendetected: the difference reflects the well‐known fact of tRNA sequence conservation.Transitions G↔A and C↔U were the majority (56%) of found substitutions. Moreimportantly, forward and backward rates of transitions were unequal and differed by 14%and 12%, correspondingly. Asymmetry of fluxes of reciprocal substitutions means that tRNAsare not at evolutionary equilibrium. Nucleotide changes are time‐irreversible and directedtowards nucleotides of weaker Watson‐Crick base‐pairing.To infer the qualitative evolutionary trajectory of nucleotide frequencies in tRNAs it isinstructive to recalculate the nucleotide changes into the transition matrix of two‐letter (S‐W) code and to apply the simplest substitution model [1]. Within the model it is assumedthat substitutions occur uniformly in time and independently of each other. Then thenucleotide frequency evolves under the following kinetic equation:n kto( 1n) kfromntUsing this equation and the forward and backward substitution rates, k to and k from , thesteady‐state nucleotide frequencies can be readily estimated (Table 1). Difference betweenthe observed and the stationary frequencies of nucleotides corresponds to the expected lossof two S‐S pairs per one tRNA in the distant future.Table 1. tRNA nucleotide content evolution: gain and loss rates, current and equilibriumfrequencies calculated from the model of independent stationary substitutions.Nucleotide Nucleotide gain\lossrate per substitutionCurrentfrequencyAsymptoticfrequencyS (G+C) ‐0.024 0.61 0.58W (A+U) 0.024 0.39 0.42Conclusion: I show that tRNAs are not in detailed evolutionary equilibrium, consistentlylosing strong (G\C) and accumulating weak (A\U) nucleotides. This may reflect underrepresentationof A and U in early tRNAs or, in other words, relaxed selection constraintfavoring G‐C pairs compared to A‐U pairs in helical regions of secondary structure. This mayalso suggest G‐C abundance in the prebiotic environment, where weak nucleotides wererare. It is well‐known that the taxon nucleotide content depends on its environmentaltemperature: therefore a universal trend found in this work supports the earlierobservations that LUCA was more thermofilic than currently living beings.Acknowledgment: The work was supported by the RAS Program "Biosphere origin andevolution" № B25.117