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Transcriptome-wide mRNA condensation precedes stress granule formation and excludes stress-induced transcripts

ABSTRACT: Stress-induced condensation of mRNA and proteins into stress granules is conserved across eukaryotes, yet the function, formation mechanisms, and relation to well-studied conserved transcriptional responses remain largely unresolved. Stress-induced exposure of ribosome-free mRNA following translational shutoff is thought to cause condensation by allowing new multivalent RNA-dependent interactions, with RNA length and associated interaction capacity driving increased condensation. Here we show that, in striking contrast, virtually all mRNA species condense in response to multiple unrelated stresses in budding yeast, length plays a minor role, and instead, stress-induced transcripts are preferentially excluded from condensates, enabling their selective translation. Using both endogenous genes and reporter constructs, we show that translation initiation blockade, rather than resulting ribosome-free RNA, causes condensation. These translation initiation-inhibited condensates (TIICs) are biochemically detectable even when stress granules, defined as microscopically visible foci, are absent or blocked. TIICs occur in unstressed yeast cells, and, during stress, grow before the appearance of visible stress granules. Stress-induced transcripts are excluded from TIICs primarily due to the timing of their expression, rather than their sequence features. Together, our results reveal a simple system by which cells redirect translational activity to newly synthesized transcripts during stress, with broad implications for cellular regulation in changing conditions.

ORGANISM(S): Saccharomyces cerevisiae

PROVIDER: GSE265963 | GEO | 2024/04/26

REPOSITORIES: GEO

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Project description:Cellular responses to maladaptive environmental changes—stresses—allow for organismal adaptation to diverse and dynamic conditions. Across the tree of life, cells upregulate a highly conserved transcriptional program in response to so-called proteotoxic stresses such as heat shock. Correspondingly, in eukaryotes, these stresses induce the formation of biomolecular condensates, clusters of mRNA and protein which are referred to as stress granules under severe stress. However, major questions remain about this stress-induced response. How conserved is the condensation response relative to the transcriptional response? How does it vary across environmental niches, and to what extent does it correspond with the conserved transcriptional response? To answer these fundamental questions, we studied the growth, transcriptional, and condensation heat-induced stress responses in three fungal species adapted to thrive in different thermal environments: cryophilic S. kudriavzevii, mesophilic S. cerevisiae, and thermotolerant K. marxianus. Here we show that transcriptional heat shock responses track each species’ evolved temperature range of growth. Further, orthologous proteins—including poly(A)-binding protein, Pab1, a core marker of stress granules—form condensates in vivo at temperatures systematically tuned to the temperature at which the organisms activate the transcriptional heat shock response and slow their growth. In vitro, purified Pab1 from each species condenses autonomously at niche-specific temperatures. Homologous mutations in Pab1 cause similar shifts in relative condensation temperature across species, and crucially, mutations which suppress condensation in vitro also reduce fitness during heat stress. Our findings indicate that stress-induced protein condensation is adaptive, conserved, integrated with the growth and transcriptional responses, and tuned to features of the cellular and organismal environment to initiate at niche-specific levels.

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Transcriptome-wide mRNA condensation precedes stress granule formation and excludes stress-induced transcripts

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