Funded by NASA’s Astrobiology Institute, a team of researchers from the Georgia Institute of Technology has retraced the development of the ribosome back almost four billion years, unlocking the chronology of its evolution from its primordial form to its present structure. The ribosome provides a site for protein synthesis in all living cells, and is therefore crucial for the existence of life. By unravelling the secrets of its formation, the new research has shed light on the pre-conditions for life, providing a window into a time that predates biology.
Ribosomes are complexes of various proteins and molecules similar to DNA called ribosomal ribonucleic acid (RNA). These are arranged into different-sized subunits that together participate in the synthesis of proteins via the linking of amino acids. The length and complexity of this RNA varies between species, and is generally longer in more advanced organisms. However, by examining 3D models of crystallized ribosomes from a number of organisms, researchers have noted that all ribosomes contain the same common core, and only vary in their outer layers. This has led them to hypothesize an “accretion model” for the evolution of ribosomes, which is best explained by using the rings in the trunk of a tree as an analogy.
As trees age, they obtain a new ring each year without altering their inner rings. As such, they retain a permanent, unchanged record of their early history, which is continually added to but never lost. Similarly, all ribosomes contain an “inner ring” that is identical to the earliest ribosome that first gave rise to life. With the march of evolution, new species have developed more complex ribosomes by adding RNA sequences to this common core, without ever losing it. As such, it is possible to observe the very first ribosomal structure in every single living thing.
Image shows the difference between RNA – which consists of a single strand of nucleotides – and the double-helix structure of DNA. Sponk via Wikimedia Commons.
However, the team behind this latest study has gone one step further by identifying the stages of formation that gave rise to this common core. In doing so, they have peered back in time to an era before the existence of life.
By looking at 3D molecules of ribosomes, they found a number “insertion fingerprints” that indicate moments of RNA expansion. By studying these fingerprints within the common core, the team was able to retrace the process by which the ribosome formed.
Publishing their findings in the Proceedings of the National Academy of Sciences, the researchers have identified six stages of development leading to the formation of the common core of modern ribosomes. Beginning with a single strand of ancestral RNA, they reveal how it became folded into the two major ribosomal subcomponents – called the large subunit (LSU) and small subunit (SSU) – which later joined together. Further developments resulted in a structure capable of stringing amino acids together, before the sixth and final phase saw the addition of a number of proteins to the surface of the core ribosome, optimizing its function.
By tracing the development of the ribosome back to its earliest form, the researchers were able to understand some of the processes that predated the arrival of life on Earth. By correlating the stages of ribosomal evolution with changing environmental conditions, the team believes they may be able to use this information to aid their search for life elsewhere in the universe.
