Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called “reverse recruitment.” An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

Glucose Signaling in Saccharomyces cerevisiae

We have characterized CaNrg1 from Candida albicans, the major fungal pathogen in humans CaNrg1 contains a zinc finger domain that is conserved in transcriptional regulators from fungi to humans It is most closely related to ScNrg1, which represses transcription in a Tup1-dependent fashion in Saccharomyces cerevisiae Inactivation of CaNrg1 in Calbicans causes filamentous and invasive growth, derepresses hypha-specific genes, increases sensitivity to some stresses and attenuates virulence A tup1 mutant displays similar phenotypes However, unlike tup1 cells, nrg1 cells can form normal hyphae, generate chlamydospores at normal rates and grow at 42°C Transcript profiling of 2002 Calbicans genes reveals that CaNrg1 represses a subset of CaTup1-regulated genes, which includes known hypha-specific genes and other virulence factors Most of these genes contain an Nrg1 response element (NRE) in their promoter CaNrg1 interacts specifically with an NRE in vitro Also, deletion of two NREs from the ALS8 promoter releases it from Nrg1-mediated repression Hence, CaNrg1 is a transcriptional repressor that appears to target CaTup1 to a distinct set of virulence-related functions, including yeast–hypha morphogenesis

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NRG1 represses yeast–hypha morphogenesis and hypha‐specific gene expression in Candida albicans

https://www.cell.com/trends/biochemical-sciences/pdf/S0968-0004(00)01592-9.pdf

Turning genes off by Ssn6–Tup1: a conserved system of transcriptional repression in eukaryotes

In response to a variety of external signals, the fungal pathogen Candida albicans undergoes a transition between ellipsoidal single cells (blastospores) and filaments composed of elongated cells attached end-to-end. Here we identify a DNA-binding protein, Nrg1, that represses filamentous growth in Candida probably by acting through the co-repressor Tup1. nrg1 mutant cells are predominantly filamentous under non-filament-inducing conditions and their colony morphology resembles that of tup1 mutants. We also identify two filament-specific genes, ECE1 and HWP1, whose transcription is repressed by Nrg1 under non-inducing conditions. These genes constitute a subset of those under Tup1 control, providing further evidence that Nrg1 acts by recruiting Tup1 to target genes. We show that growth in serum at 37°C, a potent inducer of filamentous growth, causes a reduction of NRG1 mRNA, suggesting that filamentous growth is induced by the down-regulation of NRG1. Consistent with this idea, expression of NRG1 from a non-regulated promoter partially blocks the induction of filamentous growth.

https://www.embopress.org/doi/pdf/10.1093/emboj/20.17.4753

NRG1, a repressor of filamentous growth in C.albicans, is down-regulated during filament induction.

Candida albicans, the major human fungal pathogen, undergoes a reversible morphological transition from blastospores (round budding cells) to filaments (elongated cells attached end-to-end). This t...

/pdf/induction-of-the-candida-albicans-filamentous-growth-program-1bb4lgb0vf.pdf

Induction of the Candida albicans filamentous growth program by relief of transcriptional repression: a genome-wide analysis.

Expression of genes encoding starch-degrading enzymes is regulated by glucose repression in the yeast Saccharomyces cerevisiae. We have identified a transcriptional repressor, Nrg1, in a genetic screen designed to reveal negative factors involved in the expression of STA1, which encodes a glucoamylase. The NRG1 gene encodes a 25-kDa C2H2 zinc finger protein which specifically binds to two regions in the upstream activation sequence of the STA1 gene, as judged by gel retardation and DNase I footprinting analyses. Disruption of the NRG1 gene causes a fivefold increase in the level of the STA1 transcript in the presence of glucose. The expression of NRG1 itself is inhibited in the absence of glucose. DNA-bound LexA-Nrg1 represses transcription of a target gene 10.7-fold in a glucose-dependent manner, and this repression is abolished in both ssn6 and tup1 mutants. Two-hybrid and glutathione S-transferase pull-down experiments show an interaction of Nrg1 with Ssn6 both in vivo and in vitro. These findings indicate that Nrg1 acts as a DNA-binding repressor and mediates glucose repression of the STA1 gene expression by recruiting the Ssn6-Tup1 complex.

Sang Seok Koh

Papers

Nrg1 is a transcriptional repressor for glucose repression of STA1 gene expression in Saccharomyces cerevisiae.

Cloning of RNA1 Gene from Saccharomyces cerevisiae