Thornton and others have previously shown that the evolution of modern-day proteins required permissive mutations in the past. But no one had ever investigated whether there were many or few other possible permissive mutations that could have happened, so it remained unknown how unlikely it is that evolution discovered a permissive pathway to the modern function.
To answer this question, Thornton and co-author Michael Harms, PhD, of the University of Oregon focused on the glucocorticoid receptor (GR), a key protein in the endocrine system that regulates development and stress responses in response to the hormone cortisol. They resurrected the gene for ancestral GR as it existed around 450 million years ago, before it evolved its capacity to specifically recognize cortisol. They included a handful of mutations that occurred slightly later that allowed the protein to evolve its cortisol recognition, but they left out the permissive mutations, rendering the protein nonfunctional.
Thornton and Harms then created millions of copies of this genetic template, using a method that introduced random mutations into every new copy, thus mimicking the variation that evolution could have produced in the protein under alternative scenarios. To identify permissive mutations in these "might-have-been" pathways, they engineered yeast cells that could grow only if they contained a functional GR and then introduced their "library" of mutated versions of ancestral GR into them. If any of the mutations were permissive, they would restore the GR's function and allow the yeast to grow when exposed to cortisol.
Thornton and Harms tested many thousands of variants but found none that restored the function of GR other than the historical mutations that occurred
|Contact: Kevin Jiang|
University of Chicago Medical Center