“Instant” evolution??? Prions and sci-fi adaptation
The New Scientist reports on a newly discovered ability of simple yeast to overcome hostile changes in the environment by creating a prion from a “quality control” protein known as Sup35.
Prions were discovered as the infectious agents that cause transmissible spongiform encephalopathy in a variety of mammals, including mad cow diseases and Creutzfeldt-Jakob disease. These diseases affect neural tissues by causing the misfolding of proteins that accumulate and eventually cause nerve cell death. All forms are incurable and universally fatal.
What makes prions unique is their lack of nucleic acids to direct their replication. Rather than being reproductive in the normal biological sense of assembling copies of themselves, prions recreate mechanically by a normal protein and rearranging its structure into the misfolded prion form. As such they are in effect nature’s own “nano-machines”. There is still debate as to what sort of catalyst “loads the target protein into the prion machine” though the indication is that an as yet unidentified “third-party” protein is responsible.
So what does this have to do with evolution?
To date there are two mechanisms by which genetic change that results in adaptation – mutation, a change in the DNA sequence itself, or epigenesis, or “skipping” parts of the genetic code when it is read. Prions now appear to be responsible for a third mechanism related to epigenesis where instead of blocking the reading of part of the sequence, the entire sequence is read in its entirety, allowing coding of proteins that are typically ignored.
The result is that the yeast generates a hotchpotch of brand-new proteins without changing its DNA in any way. Within that mix of new proteins could be some that are crucial for survival.
Susan Lindquist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, first saw this process, which she calls “combinatorial evolution”, in 2004, while studying lab-grown Baker’s yeast (Saccharomyces cerevisiae).
“We’ve been saying this is really cool and a way of producing new traits for years, but other people have said it’s a disease of lab yeast,” she says.
Now she’s proved the sceptics wrong by demonstrating beyond doubt that the same process happens in nature too. She has seen it at work in 255 of 700 natural yeasts she and her colleagues have studied
The result is that yeast that Lindquist grew in hostile oxygen depleted or acidic environments was able to adapt and thrive over time. Unless succeeding generations were exposed to prion destroying chemicals. Without the prions, the colonies of yeast withered and died off.
A host of questions are now being raised about what triggers prion creation, whether changes can become “permanently coded” into the genome (theoretically possible, but not observed yet) and what the implications are for higher forms than fungi.
The idea that a prion could be an “evolution accelerator” makes you look at a bunch of sci-fi plots in a whole new light.