Elongating under Stress

Control of Gene Expression via the Yeast CWI Pathway

Living cells exposed to stressful environmental situations can elicit cellular responses that guarantee maximal cell survival. Most of these responses are mediated by mitogen-activated protein kinase (MAPK) cascades, which are highly conserved from yeast to humans. Cell wall damage conditions in the yeast Saccharomyces cerevisiae elicit rescue mechanisms mainly associated with reprogramming specific transcriptional responses via the cell wall integrity (CWI) pathway. Regulation of gene expression by this pathway is coordinated by the MAPK Slt2/Mpk1, mainly via Rlm1 and, to a lesser extent, through SBF (Swi4/Swi6) transcription factors. In this review, we summarize the molecular mechanisms controlling gene expression upon cell wall stress and the role of chromatin structure in these processes. Some of these mechanisms are also discussed in the context of other stresses governed by different yeast MAPK pathways. Slt2 regulates both transcriptional initiation and elongation by interacting with chromatin at the promoter and coding regions of CWI-responsive genes but using different mechanisms for Rlm1- and SBF-dependent genes. Since MAPK pathways are very well conserved in eukaryotic cells and are essential for controlling cellular physiology, improving our knowledge regarding how they regulate gene expression could impact the future identification of novel targets for therapeutic intervention.

Recovery from heat shock requires the microRNA pathway in Caenorhabditis elegans

The heat shock response (HSR) is a highly conserved cellular process that promotes survival during stress. A hallmark of the HSR is the rapid induction of heat shock proteins (HSPs), such as HSP-70, by transcriptional activation. Once the stress is alleviated, HSPs return to near basal levels through incompletely understood mechanisms. Here, we show that the microRNA pathway acts during heat shock recovery in Caenorhabditis elegans. Depletion of the miRNA Argonaute, Argonaute Like Gene 1 (ALG-1), after an episode of heat shock resulted in decreased survival and perdurance of high hsp-70 levels. We present evidence that regulation of hsp-70 is dependent on miR-85 and sequences in the hsp-70 3’UTR that contain target sites for this miRNA. Regulation of hsp-70 by the miRNA pathway was found to be particularly important during recovery from HS, as animals that lacked miR-85 or its target sites in the hsp-70 3’UTR overexpressed HSP-70 and exhibited reduced viability. In summary, our findings show that down-regulation of hsp-70 by miR-85 after HS promotes survival, highlighting a previously unappreciated role for the miRNA pathway during recovery from stress.

In the natural world, organisms constantly face stressful conditions such as oxidative stress, pathogen infection, starvation and heat stress. While many studies have focused on the cellular response to stress, less is known about how gene expression re-sets after the stress has been ameliorated. Here, we show that the microRNA pathway plays a critical role during the recovery phase after an episode of heat shock in the nematode, Caenorhabditis elegans. Elevated temperatures induce high expression of heat shock proteins (HSPs), including HSP-70, that provide protection from the damaging effects of high heat. We found that restoration of basal levels of HSP-70 after heat shock depends on Argonaute Like Gene 1 and miR-85. Moreover, loss of miRNA-mediated repression of HSP-70 results in compromised survival following heat shock. Our study draws attention to the recovery phase of the heat shock response and highlights an important role for the microRNA pathway in re-establishing gene expression programs needed for organismal viability post stress.

Hot1 factor recruits co-activator Sub1 and elongation complex Spt4/5 to osmostress genes

Hyperosmotic stress response involves the adaptative mechanisms needed for cell survival. Under high osmolarity conditions, many stress response genes are activated by several unrelated transcription factors that are controlled by the Hog1 kinase. Osmostress transcription factor Hot1 regulates the expression of several genes involved in glycerol biosynthesis, and the presence of this transcription factor in their promoters is essential for RNApol II recruitment. The physical association between Hog1 and Hot1 activates this transcription factor and directs the RNA polymerase II localization at these promoters. We, herein, demonstrate that physical and genetic interactions exist between Hot1 and several proteins involved in transcriptional and posttranscriptional processes: for example, transcription co-activator Sub1 and elongation complex Spt4/5. The results presented in this work demonstrate that Hot1 enrichment is not detected through the coding regions of its target genes and rule out a direct role in transcription elongation. Instead, other data presented herein indicate a key function of the Hot1 transcription factor in the recruitment of these proteins to the promoter or the 5′-coding region of the genes under its control.