The origins of life on earth have been a fascination for researchers and the public alike for many years. To date, researchers have traced the arc of life back several billion years to conclude the simplest forms of life evolved over the eons into organisms of greater and greater complexity.  However, examples of these billion-year-old events have only been theorized and simulated with a lack of living examples.  Now, two studies from researchers led by Scripps Research engineer microorganisms recapitulating the key features of organisms thought to have lived billions of years ago, allowing them to explore questions about how life evolved from inanimate molecules to multicellular lifeforms.  The team states they aim to shed light on the early evolution of genetic material, including the theorized transition of life on earth using genetic molecular RNA to life forms using DNA as the primary storehouse of genetic information.  The studies are published in the journals Proceedings of the National Academy of Sciences (PNAS) and Journal of the American Chemical Society (JACS).

Previous studies involve hypotheses organisms must have relied on RNA before DNA to carry genetic information.  RNA is a molecule similar to and far less stable than DNA with the ability to catalyze chemical reactions like the production of proteins. One possibility proffered is the transition proceeded through a kind of microbial missing link, a replicating organism storing genetic information as RNA, yet how this worked or if this organism actually existed is unclear.

For the JACS study, the researchers engineered E. coli bacteria using ribonucleotides, the molecular building blocks typically used to build RNA, to partially building their DNA. Results show these engineered genomes contain up to 50% RNA and represent a new type of synthetic organism based on a  possible throwback to billions of years ago.  The team states the fact E. coli with half its genome comprised of RNA can survive and replicate is remarkable and seems to support the possibility of the existence of evolutionarily transitional organisms possessing hybrid RNA-DNA genomes. The team is now studying how the mixed genomes of their engineered E. coli function and plans to use the bacteria to explore a number of evolutionary questions.

In the paper published in PNAS, the group engineers another laboratory model for an evolutionary milestone thought to have occurred more than 1.5 billion years ago. They created a yeast dependent for energy on bacteria living inside it as a beneficial parasite or symbiote. This composite organism will allow them to investigate the ancient origins of mitochondria, microscopic bacteria-like organelles responsible for producing chemical energy within the cells of all higher organisms.

The team surmises they have used the tools of synthetic biology to engineer organisms similar to those thought to have lived billions of years ago.  For the future, the researchers state their engineered organisms will allow the global medical community to probe two key theories in the transition from the RNA world to the DNA world and the transition from prokaryotes to eukaryotes containing mitochondria.

Source: Scripps Research

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