An Organic Carbon Rich Surface and the Source of Primitive tRNA on Earth

Abstract

Modern tRNA has a shape that gives it the ability to carry out translation of genetic code to yield polypeptides (proteins). Prebiotic tRNA, if it existed, must have acquired this shape for other reasons, the most likely of which would be relative chemical stability. The nucleotide sequence of tRNA allows for multiple base-pairings that result in the peculiar shape of tRNA, but probably also leads to shielding of potential hydrolysis sites by enclosing them within the folded molecule. It may also be that attachment of an amino acid at one end of the tRNA polymer adds to chemical stability. The result would be accumulation of tRNA-like polymers in a reacting primordial soup in preference to similar sized RNA polymers lacking sequences that allow multiple base-pairing. The less stable RNA are continually broken down into smaller pieces, perhaps monomers, while the more stable polymers survive. This constraint on which RNA polymers survive and accumulate means that a relatively small amount of nucleotide material is required to synthesize all possible tRNA. This assumes that prebiotic RNA polymerization and duplication is possible, but there are laboratory experiments suggesting just that. The organic-compound-rich environment envisioned in previous chapters has enough material to allow synthesis of even multiple copies of tRNA. In order for primitive tRNA life to emerge, however, requires that enough different types of tRNA (coding for different amino acids) are somehow packaged together. This may be accomplished, in principle, by encapsulating tRNA (and other prebiotic raw materials) in organic “bags” similar to micelles or liposomes. The scenario already described provides enough additional organic material to produce enormous numbers of liposomes in addition to the tRNA. Exchange of contents between these primitive packages during fusion/breakup/refusion means that a huge number of combinations of components can take place under prebiotic conditions. The number of “experiments” is virtually uncountable, but a successful package that has the necessary components to carry out polypeptide synthesis of a “multimer” having some catalytic activity only has to happen once to form the first feedback hypercycle. Once this occurs, the package can capture energy, reproduce itself and essentially take over the available food supply (which is what the prebiotic soup would have become). The lack of selection pressure means that variations in the specific makeup of these reproducing “proto-bacteria” would rapidly spread. Some would develop slightly more efficient biochemical systems and these would survive at the expense of the less efficient, the first stage of biological, as opposed to chemical, evolution.