Scientists from the United States and South Africa have shown that short RNA nucleotide chains can spontaneously combine with each other to form large molecules. This ability is necessary for the appearance in the primary broth of compounds with enzymatic activity that store genetic information. The researchers' article was published in the journal Royal Society Open Science.
According to the hypothesis of the RNA world, the precursors of living cells were enzyme molecules (ribozymes) capable of copying themselves (replicating). It is known that such compounds are large and complex. They were supposed to be formed from smaller molecules through a process called ligation, where two nucleotide chains are linked at their ends. However, such a reaction is carried out by a separate group of enzymes (ligases), so the RNA world would be impossible without the presence of ribozymes that provide their own ligation.
The researchers used a complex ribozyme RNA polymerase R18 as an object of research. This large molecule has two active sites. One of them carries out polymerase activity, that is, synthesizes an RNA molecule from nucleotides, the other is a ligase nucleus. The specialists removed various fragments from the ribozyme, obtaining a series of small chains, all of which had a ligating center. The latter, as well as the complete R18 polymerase, were placed in an environment containing other small RNA molecules (substrates).
Scientists have evaluated the ability of molecules to self-ligate. R18 showed very low activity, but the smallest strand (R18-T4) attached a wide variety of RNA molecules to itself. With an increase in the chain length, the ligase activity decreased, and the molecule could only bind to certain substrates. According to experts, this is how ribozymes acquire the specificity characteristic of enzymes in living cells.
The results of the study showed that in the early stages of the RNA world, large molecules appeared through the fusion of ligases with various small ribonucleic chains. Initially, such compounds could not effectively perform enzymatic functions, such as self-copying, but they already had specificity for certain substrates. In the course of molecular evolution, their ability to replicate has increased.
