The chestnut blight fungus Cryphonectria parasitica and its hypovirus comprise useful model system to study the mechanisms of hypoviral
infection. We used degenerate primers based on fungal
protein kinases to isolate a gene, cppk1, which encodes a novel Ser/Thr
protein kinase of C. parasitica. The gene showed highest homology to ptk1, a Ser/Thr
protein kinase from Trichoderma reesei. The encoded
protein had a predicted mass of 70.5 kDa and a pI of 7.45. Northern blot analyses revealed that the cppk1 transcript was expressed from the beginning of culture, with a slight increase by 5 days of culture. However, its expression was specifically affected by the presence of virus, and it was transcriptionally upregulated in the fungal strain infected with the hypovirus. A
kinase assay using Escherichia coli-derived CpPK1 revealed CpPK1-specific phosphorylated
proteins with estimated masses of 50 kDa and 44 kDa. In addition, the phosphorylation of both
proteins was higher in a cell-free extract from the hypovirulent strain. The increased expression of cppk1 by the introduction of an additional copy results in a subset of viral symptoms of reduced pigmentation and conidiation in a virus-free isolate. cppk1 overexpression also causes the downregulation of
mating factor genes Mf2/1 and Mf2/2, resulting in
female sterility. The present study suggests that the hypovirus disturbs fungal signalling by transcriptional upregulation of cppk1, which results in reduced pigmentation and conidiation and
female sterility.