TRANSCRIPTIONAL REPRESSION OF SPECIFIC HOST GENES BY THE MYCOVIRUS CRYPHONECTRIA-HYPOVIRUS-1

The hypovirus CHV1, which infects the plant-pathogenic fungus Cryphone ctria parasitica, causes a distinct range of symptoms in its host that include reduced virulence expression, reduced sporulation, and reduce d pigmentation, The virus, however, has little or no effect on fungal growth in culture, The visual symptoms are associated with reduced acc umulation of a small number of host mRNAs and proteins, Four of the ho st genes encoding these down-regulated mRNAs have been characterized; they include two genes encoding a fungal sex pheromone (Vir1 and Vir2) , a gene encoding an extracellular laccase (Lac1), and a gene encoding a cell wall hydrophobin (Crp), Expression of most other host proteins appears to be unaffected by the virus. These four genes can serve as reporter genes in studies of the effect of the virus on host gene expr ession, It is hypothesized that the four genes are coordinately down-r egulated by the virus and probably are associated in a regulatory casc ade, This hypothesis was tested by measuring the relative transcriptio n rate of each gene in virus-infected and uninfected isogenic strains of the fungus by using nuclear run-on assays, The effects of the virus on transcription of these genes generally mirrored the observed effec ts of the virus on relative accumulation of the mRNAs of each gene, Al though repressed transcription cannot account for all of the effects o f the virus on mRNA accumulation of these four reporter genes, it is t he predominant effect.