The maize pathogenCochliobolus heterostrophusrequires two mitogen-activated protein kinases (MAPKs), Chk1 and Mps1, to produce normal pigmentation. Young colonies ofmps1andchk1deletion mutants have a white and autolytic appearance, which was partially rescued by a hyperosmotic environment. We isolated the transcription factor Cmr1, an ortholog ofColletotrichum lagenariumCmr1 andMagnaporthe griseaPig1, which regulates melanin biosynthesis inC. heterostrophus. Deletion ofCMR1inC. heterostrophusresulted in mutants that lacked dark pigmentation and acquired an orange-pink color. Incmr1deletion strains the expression of putative scytalone dehydratase (SCD1) and hydroxynaphthalene reductase (BRN1andBRN2) genes involved in melanin biosynthesis was undetectable, whereas expression ofPKS18, encoding a polyketide synthase, was only moderately reduced. Inchk1andmps1mutants expression ofPKS18,SCD1,BRN1,BRN2, and the transcription factorCMR1itself was very low in young colonies, slightly up-regulated in aging colonies, and significantly induced in hyperosmotic conditions, compared to invariably high expression in the wild type. These findings indicate that two MAPKs, Chk1 and Mps1, affect Cmr1 at the transcriptional level and this influence is partially overridden in stress conditions including aging culture and hyperosmotic environment. Surprisingly, we found that theCMR1gene was transcribed in both sense and antisense directions, apparently producing mRNA as well as a long noncoding RNA transcript. Expression of the antisenseCMR1was also Chk1 and Mps1 dependent. Analysis of chromosomal location of the melanin biosynthesis genes inC. heterostrophusresulted in identification of a small gene cluster comprisingBRN1,CMR1, andPKS18. Since expression of all three genes depends on Chk1 and Mps1 MAPKs, we suggest their possible epigenetic regulation.

Significance: The thioredoxin (TRX) and glutathione (GSH) pathways are universally conserved thiol-reductase systems that drive an array of cellular functions involving reversible disulfide formation. Here we consider these pathways in Saccharomyces cerevisiae, focusing on their cell compartment-specific functions, as well as the mechanisms that explain extreme differences of redox states between compartments. Recent Advances: Recent work leads to a model in which the yeast TRX and GSH pathways are not redundant, in contrast to Escherichia coli. The cytosol possesses full sets of both pathways, of which the TRX pathway is dominant, while the GSH pathway acts as back up of the former. The mitochondrial matrix also possesses entire sets of both pathways, in which the GSH pathway has major role in redox control. In both compartments, GSH has also nonredox functions in iron metabolism, essential for viability. The endoplasmic reticulum (ER) and mitochondrial intermembrane space (IMS) are sites of intense thiol oxidation, but except GSH lack thiol-reductase pathways. Critical Issues: What are the thiol-redox links between compartments? Mitochondria are totally independent, and insulated from the other compartments. The cytosol is also totally independent, but also provides reducing power to the ER and IMS, possibly by ways of reduced and oxidized GSH entering and exiting these compartments. Future Directions: Identifying the mechanisms regulating fluxes of GSH and oxidized glutathione between cytosol and ER, IMS, and possibly also peroxisomes, vacuole is needed to establish the proposed model of eukaryotic thiol-redox homeostasis, which should facilitate exploration of this system in mammals and plants. Antioxid. Redox Signal. 18, 1699–1711.

The endoplasmic reticulum (ER) is a multifunctional organelle that constitutes the entry into the secretory pathway. The ER contributes to the maintenance of cellular calcium homeostasis, lipid synthesis and productive secretory, and transmembrane protein folding. Physiological, chemical, and pathological factors that compromise ER homeostasis lead to endoplasmic reticulum stress (ER stress). To cope with this situation, cells activate an adaptive signaling pathway termed the unfolded protein response (UPR) that aims at restoring ER homeostasis. The UPR is transduced through post‐translational, translational, post‐transcriptional, and transcriptional mechanisms initiated by three ER‐resident sensors, inositol‐requiring protein 1α, activating transcription factor 6α, and PRKR‐like endoplasmic reticulum kinase. Determining the in and out of ER homeostasis control and UPR activation still represents a challenge for the community. Hence, standardized criteria and methodologies need to be proposed for monitoring ER homeostasis and ER stress in different model systems. Here, we summarize the pathways that are activated during ER stress and provide approaches aimed at assess ER homeostasis and stress in vitro and in vivo mammalian systems that can be used by researchers to plan and interpret experiments. We recommend the use of multiple assays to verify ER stress because no individual assay is guaranteed to be the most appropriate one.