Our genetic code still holds mysteries for us to discover.
Sawsan Wehbi, Genetics Ph.D. candidate at the University of Arizona, takes a look.
I am a fourth year PhD candidate majoring in genetics and minoring in astrobiology. I am interested in the origins of life on Earth. My research focuses on early life proteins and the evolution of the genetic code. I am currently funded by the NASA Future Investigators in NASA Earth and Space Science and Technology (FINESST) grant.
Resolving the Order of Amino Acid Recruitment Into the Genetic Code
For years, scientists thought that twenty amino acids were added to the genetic code in a specific order. One that was largely based on abiotic synthesis metrics like the famous Urey-Miller experiment. But this approach had limitations. For example, the Urey-Miller experiment didn’t include sulfur, excluding the possibility of forming any sulfur-containing amino acids and consequently labelling them as later additions to the code. This is why we wanted to take a fresh look at this question, using actual biological evidence rather than just chemical assumptions.
So, instead of looking at early Earth chemistry, we studied some of the oldest protein sequences—ones that go all the way back to LUCA, the last universal common ancestor of all life. By analyzing how amino acid usage changed over time, we could track when different amino acids were added to the genetic code.
What we found was surprising! Amino acids like cysteine and methionine, which contain sulfur, were added much earlier than we thought. Histidine, a key amino acid for binding metals, also showed up earlier than we assumed. That means early life may have been using sulfur-based chemistry and metal-binding proteins far sooner than expected.
Even more interesting, some of the most ancient proteins—ones that existed before LUCA even formed had very different amino acid compositions—having a higher usage of ring-containing amino acids. We think this indicates that there might have been alternative genetic codes before the one we use today!
These discoveries don’t just rewrite the story of early life on Earth; they also have big implications for how we search for life beyond our planet. If life elsewhere followed a different path, we might need to rethink what we’re looking for.
Read More:
[PNAS] – Order of amino acid recruitment into the genetic code resolved by last universal common ancestor’s protein domains
