Granules harboring translationally active mRNAs provide a platform for P-body formation following stress.

The localization of mRNA to defined cytoplasmic sites in eukaryotic cells not only allows localized protein production but also determines the fate of mRNAs. For instance, translationally repressed mRNAs localize to P-bodies and stress granules where their decay and storage, respectively, are directed. Here, we find that several mRNAs are localized to granules in unstressed, actively growing cells. These granules play a key role in the stress-dependent formation of P-bodies. Specific glycolytic mRNAs are colocalized in multiple granules per cell, which aggregate during P-body formation. Such aggregation is still observed under conditions or in mutants where P-bodies do not form. In unstressed cells, the mRNA granules appear associated with active translation; this might enable a coregulation of protein expression from the same pathways or complexes. Parallels can be drawn between this coregulation and the advantage of operons in prokaryotic systems.

mRNA localization to P-bodies in yeast is bi-phasic with many mRNAs captured in a late Bfr1p-dependent wave.

The relocalization of translationally repressed mRNAs to mRNA processing bodies Pbodies is a key consequence of cellular stress across many systems. Pbodies harbor mRNA degradation components and are implicated in mRNA decay, but the relative timing and control of mRNA relocalization to Pbodies is poorly understood. We used the MS2GFP system to follow the movement of specific endogenous mRNAs in live Saccharomyces cerevisiae cells after nutritional stress. It appears that the relocalization of mRNA to Pbodies after stress is biphasic some mRNAs are present early, whereas others are recruited much later concomitant with recruitment of translation initiation factors, such as eIF4E. We also find that Bfr1p is a latephaselocalizing Pbody protein that is important for the delayed entry of certain mRNAS to Pbodies. Therefore, for the mRNAs tested, relocalization to Pbodies varies both in terms of the kinetics and factor requirements. This work highlights a potential new regulatory juncture in gene expression that would facilitate the overall rationalization of protein content required for adaptation to stress.

Adaptation to stress in yeast: to translate or not?

For most eukaryotic organisms, including Saccharomyces cerevisiae, the rapid inhibition of protein synthesis forms part of a response to stress. In order to balance the changing conditions, precise stress-specific alterations to the cell's proteome are required. Therefore, in the background of a global down-regulation in protein synthesis, specific proteins are induced. Given the level of plasticity required to enable stress-specific alterations of this kind, it is surprising that the mechanisms of translational regulation are not more diverse. In the present review, we summarize the impact of stress on translation initiation, highlighting both the similarities and distinctions between various stress responses. Finally, we speculate as to how yeast cells generate stress-responsive programmes of protein production when regulation is focused on the same steps in the translation pathway.