Network of nutrient-sensing pathways and a conserved kinase cascade integrate osmolarity and carbon sensing in Neurospora crassa

Abstract

Identifying nutrients available in the environment and utilizing them in the most efficient manner is a challenge common to all organisms. The model filamentous fungus Neurospora crassa is capable of utilizing a variety of carbohydrates, from simple sugars to the complex carbohydrates found in plant cell walls. The zinc binuclear cluster transcription factor CLR-1 is necessary for utilization of cellulose, a major, recalcitrant component of the plant cell wall; however, expression of clr-1 in the absence of an inducer is not sufficient to induce cellulase gene expression. We performed a screen for unidentified actors in the cellulose-response pathway and identified a gene encoding a hypothetical protein (clr-3) that is required for repression of CLR-1 activity in the absence of an inducer. Using clr-3 mutants, we implicated the hyperosmotic-response pathway in the tunable regulation of glycosyl hydrolase production in response to changes in osmolarity. The role of the hyperosmotic-response pathway in nutrient sensing may indicate that cells use osmolarity as a proxy for the presence of free sugar in their environment. These signaling pathways form a nutrient-sensing network that allows N. crassa cells to tightly regulate gene expression in response to environmental conditions. MAP kinase cascade | carbon metabolism | hyperosmotic response | carbon catabolite repression | plant biomass degradation

Broad substrate-specific phosphorylation events are associated with the initial stage of plant cell wall recognition inNeurospora crassa

“…Although not a transcription factor, CLR-3 (NCU05846) is still highly relevant for cellulase regulation (Huberman et al, 2017) and was found to be phosphorylated in two regions and three potential sites (Supplementary Material 2). Interestingly, phosphorylation of T713 within a predicted casein kinase II phosphorylation site was specific for the absence of carbon.…”

“…The Mitogen-Activated Protein Kinase (MAP kinase) pathway, for example, represents a prototype of a signal distribution channel with phosphorylation being the major switch. In N. crassa, MAP kinases are known to be involved in cell-to-cell communication (Fischer et al, 2018), circadian regulation of cellular processes (Bennett et al, 2013; Lamb et al, 2012), phosphate signaling (Gras et al, 2013) as well as sensing of osmolarity and carbon (Huberman et al, 2017). Phosphorylation/dephosphorylation reactions involved in nutrient-sensing are furthermore central to the cyclic AMP (cAMP) signal transduction pathway (Aichinger et al, 1999; Ficarro et al, 2002; Lengeler et al, 2000).…”

Studies of Cellulose and Starch Utilization and the Regulatory Mechanisms of Related Enzymes in Fungi

“…Clr-1 is another transcription factor that is necessary for cellulose utilization. However, its regulation of cellulase gene expression is strongly influenced by inducers: Clr-1 cannot induce the expression of the cellulase gene without an inducer [102]. In the presence of an inducer, such as cellobiose, a degradation product of cellulose, Clr-1 induces the expression of some genes containing β-glucosidases and Clr-2, a transcription regulator [103].…”

“…Through the screening of mutants in the absence of an inducer, a Clr-3 that inhibits the activity of Clr-1 was identified in N. crassa. It was found that the deletion of the clr-3 gene led to cellulase gene expression of Clr-1-dependent even without an inducer [102,105].…”

CLR‐4, a novel conserved transcription factor for cellulase gene expression in ascomycete fungi

“…Huberman et al . () recently identified a new repressor, CLR‐3, that acts through CLR‐1 to repress the cellulolytic response. Their results also showed that the hyperosmotic response pathway regulates the transcription of cellulase genes in N. crassa (Huberman et al ., ).…”