Possible involvement of nitric oxide and reactive oxygen species in glucose deprivation-induced activation of transcription factor rst2

Molecular mechanisms and highly functional development for stress tolerance of the yeast Saccharomyces cerevisiae

In response to environmental stress, microorganisms adapt to drastic changes while exerting cellular functions by controlling gene expression, metabolic pathways, enzyme activities, and protein-protein interactions. Microbial cells that undergo a fermentation process are subjected to stresses, such as high temperature, freezing, drying, changes in pH and osmotic pressure, and organic solvents. Combinations of these stresses that continue over long terms often inhibit cells' growth and lead to their death, markedly limiting the useful functions of microorganisms (eg their fermentation ability). Thus, high stress tolerance of cells is required to improve productivity and add value to fermented/brewed foods and biofuels. This review focuses on stress tolerance mechanisms, including l-proline/l-arginine metabolism, ubiquitin system, and transcription factors, and the functional development of the yeast Saccharomyces cerevisiae, which has been used not only in basic science as a model of higher eukaryotes but also in fermentation processes for making alcoholic beverages, food products, and bioethanol.

Role of mitochondrial complex III/IV in the activation of transcription factor Rst2 in Schizosaccharomyces pombe

Mitochondria play essential roles in eukaryotic cells for glucose metabolism to produce ATP. In Schizosaccharomyces pombe, transcription factor Rst2 can be activated upon glucose deprivation. However, the link between Rst2 and mitochondrial function remains elusive. Here, we monitored Rst2 transcriptional activity in living cells using a Renilla luciferase reporter system, and found that inhibition of mitochondrial complex III/IV caused cells to produce reactive oxygen species (ROS) and nitric oxide (NO), which in turn activated Rst2. Furthermore, Rst2-GFP was observed to translocate from cytoplasm to nucleus upon mitochondrial complex III/IV inhibitors treatment, and deletion of genes associated with complex III/IV resulted in delayed process of Rst2-GFP nuclear exportation under glucose-rich condition. In particular, we found that Rst2 was phosphorylated following the treatment of complex III/IV inhibitors or SNAP. Altogether, our findings suggest that mitochondrial complex III/IV participates in the activation of Rst2 through ROS and NO generation in Schizosaccharomyces pombe.

A genome-wide analysis of carbon catabolite repression in Schizosaccharomyces pombe

BACKGROUND

Optimal glucose metabolism is central to the growth and development of cells. In microbial eukaryotes, carbon catabolite repression (CCR) mediates the preferential utilization of glucose, primarily by repressing alternate carbon source utilization. In fission yeast, CCR is mediated by transcriptional repressors Scr1 and the Tup/Ssn6 complex, with the Rst2 transcription factor important for activation of gluconeogenesis and sexual differentiation genes upon derepression. Through genetic and genome-wide methods, this study aimed to comprehensively characterize CCR in fission yeast by identifying the genes and biological processes that are regulated by Scr1, Tup/Ssn6 and Rst2, the core CCR machinery.

RESULTS

The transcriptional response of fission yeast to glucose-sufficient or glucose-deficient growth conditions in wild type and CCR mutant cells was determined by RNA-seq and ChIP-seq. Scr1 was found to regulate genes involved in carbon metabolism, hexose uptake, gluconeogenesis and the TCA cycle. Surprisingly, a role for Scr1 in the suppression of sexual differentiation was also identified, as homothallic scr1 deletion mutants showed ectopic meiosis in carbon and nitrogen rich conditions. ChIP-seq characterised the targets of Tup/Ssn6 and Rst2 identifying regulatory roles within and independent of CCR. Finally, a subset of genes bound by all three factors was identified, implying that regulation of certain loci may be modulated in a competitive fashion between the Scr1, Tup/Ssn6 repressors and the Rst2 activator.

CONCLUSIONS

By identifying the genes directly and indirectly regulated by Scr1, Tup/Ssn6 and Rst2, this study comprehensively defined the gene regulatory networks of CCR in fission yeast and revealed the transcriptional complexities governing this system.

Nitric Oxide Signalling in Yeast

Nitric oxide (NO) is a cellular signalling molecule widely conserved among organisms, including microorganisms such as bacteria, yeasts, and fungi, and higher eukaryotes such as plants and mammals. NO is mainly produced by the activities of NO synthase (NOS) or nitrite reductase (NIR). There are several NO detoxification systems, including NO dioxygenase (NOD) and S-nitrosoglutathione reductase (GSNOR). NO homeostasis, based on the balance between NO synthesis and degradation, is important for regulating its physiological functions, since an excess of NO causes nitrosative stress due to the high reactivity of NO and NO-derived compounds. In yeast, NO may be involved in stress responses, but the role of NO and the mechanism underlying NO signalling are poorly understood due to the lack of mammalian NOS orthologs in the yeast genome. NOS and NIR activities have been observed in yeast cells, but the gene-encoding NOS and the mechanism by which NO production is catalysed by NIR remain unclear. On the other hand, yeast cells employ NOD and GSNOR to maintain intracellular redox balance following endogenous NO production, treatment with exogenous NO, or exposure to environmental stresses. This article reviews NO metabolism (synthesis, degradation) and its regulation in yeast. The physiological roles of NO in yeast, including the oxidative stress response, are also discussed. Such investigations into NO signalling are essential for understanding how NO modulates the genetics and physiology of yeast. In addition to being responsible for the pathology and pharmacology of various degenerative diseases, NO signalling may be a potential target for the construction and engineering of industrial yeast strains.