A Model for the Origin of the First mRNAs.

I present a model for the evolution of the genetic code that seems to predict, in a totally natural way, the origin of the first mRNAs. In particular, the model--bestowing to peptidated-RNAs the major catalytic role in the phase that triggered the genetic code origin--suggests that interactions between peptidated-RNAs led to the synthesis of these ancestral catalysts. Within every group of these interactions, a pre-mRNA molecule evolved that was able to direct all interactions between peptidated-RNAs of that particular group. This represented an improvement in the coding of these interactions compared to the interaction groups that did not evolve these pre-mRNAs. This would represent a natural and intrinsic tendency. Therefore, these molecules of pre-mRNAs were positively selected because they improved the synthesis of the catalysts through this first form of coding of interactions among peptidated-RNAs. Thus, according to the model were the pairings--involving a base number greater than three (ennuplet code)-between peptidated-RNAs and pre-mRNAs that would represent the first form of the genetic code. The evolution of this ennuplet code to the triplet code might have been simply triggered by the natural tendency to make the reading module-that is the interactions between peptidated-RNAs and pre-mRNAs--of the different ennuplets to the triplet uniform, because in this way the heterogeneity existing in interactions between the aminoacylated or peptidated-RNAs and pre-mRNAs was eliminated. That is to say, there might have been the natural tendency toward the triplets because these would have made these interactions more efficient, given that the ennuplets were at least more cumbersome and therefore less economic and with an inferior adaptive value; and also because the triplets would represent the simpler choice among that available given that the doublets would have codified too few meanings and quartets instead too many. Therefore, the genetic code would result from a very long era of interactions among peptidated-RNAs under the continuous and fundamental selective pressure for improving catalysts' syntheses and thus catalysis. The model is strongly corroborated by the explanation that the tmRNA molecule (transfer-messenger RNA) would seem to be the very molecule of pre-mRNAs that the model predicts. In other words, the tmRNA would be the molecular fossil of the evolutionary stages that led to the appearance of the first mRNAs.