The Curie Institute, Paris, France.

An expert in chromosome organization considers yeast in a new light.

As somebody who studies how DNA is packaged so that it fits inside the nucleus, and how this protein parcelling adds to the information held in the sequence of DNA bases, my work has focused on frogs and mammals. Brewers’ yeast (Saccharomyces cerevisiae) is one of the simplest organisms with nuclei. It has proved useful to researchers like me when considering subtle influences on gene expression that are also found in higher organisms.

But we have not found a yeast ‘counterpart’ for some mechanisms, such as those that rely on RNA to regulate gene expression. One example is RNA interference, by which genes are ‘silenced’ through destruction of the messenger RNA molecules that would otherwise convey protein ‘recipes’ from the nucleus to the cytoplasm. But this does not rule out similar effects on gene expression by other means, as Françoise Stutz and her colleagues at the University of Geneva in Switzerland have found (J. Camblong et al. Cell 131, 706–717; 2007).

This team stumbled across silencing of a different sort when they left plates of yeast to divide for varying amounts of time. They found that older yeast cells expressed a gene called PHO84 less than did younger cells, and that as the amount of mRNA encoding the Pho84 protein decreased, the level of an antisense (or mirror-image) version of this mRNA increased. A series of experiments led them to propose a mechanistic model in which tuning the RNA degradation machinery stabilizes the antisense transcripts, promoting modifications of chromatin — the DNA–protein complexes that make up chromosomes — and, in turn, regulating gene expression.

No one yet knows how common this effect is in yeast, nor whether it occurs in more complex lifeforms; but this paper does serve as a lesson to revisit our assumptions.