Altered transcriptional response to nutrient availability in hypovirus-infected chestnut blight fungus

The gene lac-1, encoding the enzyme laccase, is one of several genes of the chestnut blight fungus, Cryphonectria parasitica, that are suppressed by virulence-attenuating mycoviruses of the hypovirus group. Two antagonistic regulatory pathways have been shown to govern the activity of the lac-1 promoter: a positive pathway that stimulates transcription and a negative pathway that represses transcription. We now report that these two regulatory pathways respond independently to specific changes in the nutritional environment. These newly defined conditions were used to confirm that a hypovirus suppresses the activity of the positive regulatory pathway, and to implicate calmodulin and calcineurin as components of the signal transduction cascades regulating lac-1 transcription. Significantly, lac-1 transcript accumulation was shown to be affected by amino acid availability. Further analysis revealed that transcriptional repression mediated by the negative regulatory pathway is relieved under conditions of amino acid deprivation. Thus, by blocking the positive pathway, hypovirus infection prevents increased lac-1 transcript accumulation in response to amino acid deficiency. These observations are consistent with the hypothesis that hypoviruses alter the transcriptional response of the host fungus to changes in nutrient availability.