Prions（小蛋白粒） , the much-maligned proteins most commonly known for causing "mad cow" disease, are commonly used in yeast to produce beneficial traits in the wild. Moreover, such traits can be passed on to subsequent generations and eventually become "hard-wired" into the genome, contributing to evolutionary change. Prions were first found to produce heritable new traits more than a decade ago in laboratory studies of simple baker's yeast. The key discovery then was that some proteins could spontaneously switch from a normal shape into a self-perpetuating prion conformation（构造，一致） . The switch to the prion state alters protein function, which can result in the appearance of new traits, some helpful, some detrimental. Sophisticated cellular machinery ensures that replicating prion templates are chopped into pieces that can be passed to daughter cells during cell division. Importantly, the rate at which proteins switch into and out of the prion state increases in response to environmental stress, suggesting that they are part of an inherent survival mechanism that helps yeasts adapt to changes in their surroundings.

Yet, as compelling as the case for this protein-based mechanism of inheritance is, its biological significance has been hotly debated for one key reason: prions capable of modifying phenotypes（外表型） have never been found in nature. Until now.

In a massive undertaking, Whitehead Institute scientists have tested nearly 700 wild yeast strains isolated from diverse environments for the presence of known and unknown prion elements, finding them in one third of all strains. All the prions appear capable of creating diverse new traits, nearly half of which are beneficial. These unexpected findings, reported in this week's edition of the journal Nature, stand as strong evidence against the common argument that prions are merely yeast "diseases" or rare artifacts（人工产品） of laboratory culture.

"A huge amount of effort has gone into studying this paradigm-shifting mode of inheritance, but with no real understanding of whether it's genuinely important biologically," says Daniel Jarosz, co-first author of the Nature paper and a postdoctoral researcher in the lab of Whitehead Member Susan Lindquist. "Now it seems clear they do influence the way natural yeasts cope with changing environments and evolve in response to stress."