Network of nutrient-sensing pathways and a conserved kinase cascade integrate osmolarity and carbon sensing in <i>Neurospora crassa</i>.

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 <i>Neurospora crassa</i> 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 <i>clr-1</i> 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 (<i>clr-3</i>) that is required for repression of CLR-1 activity in the absence of an inducer. Using <i>clr-3</i> 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 <i>N</i><i>crassa</i> cells to tightly regulate gene expression in response to environmental conditions.