According to an idea known as the RNA world hypothesis, RNA-based life-forms preceded modern ones that use DNA and protein. RNA, a molecular cousin of DNA, still plays roles in modern cells but does not serve as the primary genetic material. By comparison, primordial species used RNA to store genetic information and to catalyze reactions as stand-in enzymes.
Scientists originally thought that only proteins were varied enough to assemble into large structures, but a new paper has demonstrated that RNA — though more limited in its variety — also has the capacity to form these large configurations. The research was posted to the preprint server bioRxiv July 1 and has not been peer-reviewed yet.
Huang and his colleagues had hypothesized that RNA molecules could link together if they possessed sequences that fold into "kissing stem loops." This occurs when an RNA strand folds over on itself, forming a structure that resembles a loop in a shoelace. If loops from different RNAs bond together, or "kiss," the molecules could link up and form larger complexes, the researchers proposed.
They discovered that some of the RNA molecules formed long filaments. These resembled protein-based filaments such as the cellular cytoskeleton, a scaffold that participates in many functions, including shaping and moving the cell.
RNA structures assemble into icosahedra as large as protein-based virus capsids. (Image credit: Lin Huang)
This work demonstrates that RNA had the capacity to assemble into these elaborate structures during the RNA world, Huang said, but that doesn't prove it actually happened.
If scientists could recreate these environmental conditions at the dawn of life, such as high temperatures and low pH, and still observe that these structures take shape, that would strengthen the theory that they could have been present in the RNA world, she said.
Related storiesAlthough the RNA cages and filaments were large, Huang's team generated them using only short RNA strands, each no longer than 200 subunits. Medvegy said long RNAs are susceptible to breaking, so if short strands can assemble into these structures, that provides more promise that these multi-tiered molecules could have formed in the RNA world.
Beyond providing insight into life's beginnings, these RNA cages could have potential applications in biotechnology, Huang thinks. Efforts are underway to use DNA folded into "DNA origami" to deliver drugs into cells, and Huang thinks DNA's older cousin, RNA, could one day play a similar role in medicine.
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