The Role of Reverse Transcriptase in the Origin of Life

It has been suggested that RNA polymerase ribozyme displaying reverse transcriptase and integrase activities has played a vital role in the origin of life on Earth. Here, we present a hypothesis that formation of universal ancestral units of all living organisms - retroelements - in the evolution was mediated by reverse transcriptase. The propensity of retroelements to mutations and their insertion capacity have formed a basis for the origin of complex DNA structures - primary genomes - that have given rise to archaea, eukaryotes, bacteria, and viruses. Conserved properties of retroelements have been preserved throughout the evolution; their modifications have facilitated the emergence of mechanisms for the interactions between proteins and nucleic acids. Life has evolved due to insertional mutagenesis and competition of autonomously replicating polynucleotides that allowed to preserve structures with adaptive properties. We hypothesize that natural selection of mechanisms for the defense against insertions based on the ribonuclease activity of reverse transcriptase ribozyme has led to the emergence of all universal enzymatic systems for the processing of RNA molecules. These systems have been and still remain the key sources of structural and functional transformations of genomes in the course of evolution. The data presented in this review suggest that the process of translation, which unifies the nucleic acid and protein worlds, has developed as a modification of the defense mechanisms against insertions. Polypeptides formed by this defense system have potentiated the activity of ribozymes in the composition of ribonucleoproteins (RNPs) and even functionally replaced them as more efficient catalysts of biological reactions. Here, we analyze the mechanisms of retroelement involvement in the structural and regulatory transformations of eukaryotic genomes supposedly reflecting the adaptive principles that had originated during the beginning of life on Earth. Simultaneously with the evolution of existing proteins, retroelements have served as sources of new ribozymes, such as long non-coding RNAs. These ribozymes can function in complexes with proteins in the composition of RNPs, as well as display independent catalytic and translational activities; their genes have a potential for the transformation into protein-coding genes. Hence, the conserved principles of RNA, DNA, and proteins interregulation formed at the time of life origin on Earth have been used throughout the evolution.