Understanding the principles that led to the current complexity of the genetic code is a central question in evolution. Expansion of the genetic code required the selection of new transfer RNAs (tRNAs) with specific recognition signals that allowed them to be matured, modified, aminoacylated, and processed by the ribosome without compromising the fidelity or efficiency of protein synthesis. We show that saturation of recognition signals blocks the emergence of new tRNA identities and that the rate of nucleotide substitutions in tRNAs is higher in species with fewer tRNA genes. We propose that the growth of the genetic code stalled because a limit was reached in the number of identity elements that can be effectively used in the tRNA structure.

This work was supported in part by the Spanish Ministry of Economy and Competitiveness (grants BIO2012-32200, Sev-2012-0208, and BIO2012-32868 to L.R.d.P., F.A.K., and M.O., respectively) and by the Catalan Government (grants 2014-SGR-0771, 2014-SGR-0974, and 2014-SGR-0134 to L.R.d.P., F.A.K., and M.O., respectively). This work was also supported by the Howard Hughes Medical Institute International Early Career Scientist Program (55007424), by a European Research Council (ERC) Starting Grant (335980_EinME to F.K.), and by a grant from the ERC (ERC_SimDNA to M.O). A.G.T. and C.B. are funded by the Spanish Ministry of Economy and Competitiveness (FPDI-2013-17742 and BES-2013-064004, respectively).