Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 72

Discussions about viruses and their simplicity or complexity form bridges that were once hard to imagine. Large viruses partially overlap with cellular mechanisms and their upper limit appears to be life. But which is the lower limit for viruses? The smallest viruses discussed here are less representative for the lower limit of infectious mechanisms. The lower limit is represented by different RNA fragments or different proteins such as prions. Prions are misfolded proteins with the ability to transmit their misfolded shape onto correctly folded proteins of the same type (please see the mad cow disease). Prion mechanisms are, perhaps, less relevant to the occurrence of life on Earth and will not be discussed here. However, the mechanisms related to self-replicating proteins represent one of the competing hypotheses for the preorigins of life. For instance, amyloid fibers arise spontaneously from amino acids under prebiotic conditions. Thus, amyloid catalysts may have played an important role in prebiotic molecular evolution [225]. In the RNA world, the current bet for the origin of life on Earth is represented by catalytic RNAs. Examples of short RNA fragments with different properties are found in many varied and distant cases throughout the scientific literature. For instance, RNA fragments of several hundred nucleotides called “viroids” are the smallest infectious pathogens [226]. Viroids were first observed in the roots of Solanum tuberosum (potatoes) by Theodor Otto Diener in 1971 [226]. The ssRNA circular structure of viroids or viroid-like satellite RNAs lacks the presence of any genes and stands somewhere in between “nothing” and RNA viruses [216]. Apparently, RNAs are the only biological macromolecules that can function both as genotype and phenotype [227]. Some viroids and viroid-like RNAs exhibit catalytic properties that allow self-cleavage and ligation [228]. This catalytic property links the opportunistic RNAs to self-splicing introns (Group I introns). Group I introns are found in protein coding genes of bacteria and their phages, nuclear ribosomal RNA (rRNA) genes, mitochondrial mRNA and rRNA genes, chloroplast transfer RNA (tRNA) genes, and so on [229–231]. In 1981, Theodor Otto Diener asked the question: Are viroids escaped introns? [232]. A small fraction of the nuclear group I introns have the potential of being mobile elements [233]. Of course, today one can ask a complementary question: Are introns some distant viroid-like RNAs introduced into the genome of different organisms through DNA intermediates? It is likely that noncoding RNAs were the indirect source for all introns [227]. These speculations place the early opportunistic catalytic RNAs at the point of origin for the eukaryotic proteome diversity. In conclusion, viroid-like molecules could have been directly implicated in the occurrence of life on Earth. It is reasonable to believe that an intersection between self-replicating proteins and catalytic RNAs has probably led to some truly rudimentary precellular forms of life. Thus, it can be speculated here that in the prebiotic period there could have been two rudimentary life forms, which gradually merged to form the Last Universal Common Ancestor (LUCA) population. Please note that “viroids” are short ssRNAs and “virions” are virus particles.