Viral Repression of Fungal Pheromone Precursor Gene Expression

Abstract

Biological control of chestnut blight caused by the filamentous ascomycete Cryphonectria parasitica can be achieved with a virus that infects this fungus. This hypovirus causes a perturbation of fungal development that results in low virulence (hypovirulence), poor asexual sporulation, and female infertility without affecting fungal growth in culture. At the molecular level, the virus is known to affect the transcription of a number of fungal genes. Two of these genes, Vir1 and Vir2, produce abundant transcripts in noninfected strains of the fungus, but the transcripts are not detectable in virus-infected strains. We report here that these two genes encode the pheromone precursors of the Mat-2 mating type of the fungus; consequently, these genes have been renamed Mf2/1 and Mf2/2. To determine if the virus affects the mating systems of both mating types of this fungus, the pheromone precursor gene, Mf1/1, of a Mat-1 strain was cloned and likewise was found to be repressed in virus-infected strains. The suppression of transcription of the pheromone precursor genes of this fungus could be the cause of the mating defect of infected strains of the fungus. Although published reports suggest that a G␣ i subunit may be involved in this regulation, our results do not support this hypothesis. The prepropheromone encoded by Mf1/1 is structurally similar to that of the prepro-p-factor of Schizosaccharomyces pombe. This is the first description of the complete set of pheromone precursor genes encoded by a filamentous ascomycete.Chestnut blight, caused by the ascomycete Cryphonectria parasitica, is one of the most devastating plant diseases in recorded history. Because of this disease, only occasional root sprouts grow where the American chestnut (Castanea dentata) was once dominant in eastern North America. A similar fate for the European chestnut (C. sativa) in European forests and orchards was prevented by a naturally occurring biological control of the fungus. The basis of this biological control is infection of C. parasitica by double-stranded RNA viruses which reduce the virulence of the fungus, causing a condition termed hypovirulence. The effect of the virus on reducing the virulence of this plant pathogen is the basis of an effective biological control of chestnut blight (1,7,13,16,24,25).One of these hypoviruses, CHV1-713, causes no detectable effects on the growth rate of its host in culture but perturbs normal developmental processes such as sporulation and virulence. The visible symptoms of CHV1-infected colonies growing on agar are a white rather than orange colony color and poor asexual sporulation, providing a useful phenotype for the study of this virus. The sexual cycle of the fungus is also perturbed by the virus, with the female parent being sterile. At the molecular level, the virus causes transcriptional down-regulation of a number of host genes (10). The cloning and identification of the function of some of these genes have provided us with a group of molecular markers of virus infection. We have previo...

The impact of parasitism on resource allocation in a fungal host: the case of Cryphonectria parasitica and its mycovirus, Cryphonectria Hypovirus 1

“…Any shift in the balance of resources that increases C. parasitica asexual sporulation rate would then benefit CHV1. For instance, the sexual castration that has been reported in some hypovirulent C. parasitica isolates (Zhang, Baasiri, & Van Alfen, 1998) could be interpreted as redirection of resources away from traits not benefiting viral fitness. The observation of a positive relationship between somatic growth and asexual reproduction in hypovirulent isolates could then suggest that CHV1 when infecting C. parasitica releases in its host a constraint between two important life‐history traits—that we found to be negatively correlated in uninfected isolates—in order to benefit to its own fitness.…”

Biogenesis of the Saccharomyces cerevisiae Pheromone a -Factor, from Yeast Mating to Human Disease

“…Species of the Ascomycetes for which putative prenylated precursors have been reported in the literature include the following: the fission yeast Schizosaccharomyces pombe (69)(70)(71)266), the human pathogen Candida albicans (79), the filamentous fungi Neurospora crassa (31,138) and Sordaria macrospora (176,205), the chestnut blight fungus Cryphonectria parasitica (282), the rice blast pathogen Magnathorpe grisea (242), the white blight pathogen Gibberella zeae (139,158), and the cellulase producer Hypocrea jecorina (238). For H. jecorina, the precursor is unusual, sharing commonalities with S. cerevisiae a-factor and ␣-factor precursors.…”

Characterization of the ERK homologue CpMK2 from the chestnut blight fungus Cryphonectria parasitica

“…Genes of two classes of pheromones were identified recently from three heterothallic filamentous fungi, C. parasitica, M. grisea and Neurospora crassa, and from the homothallic fungus Sordaria macrospora (Zhang et al, 1998;Shen et al, 1999;Poggeler, 2000;Bobrowicz et al, 2002;Turina et al, 2003). The first complete set of pheromone precursor genes to be identified in a filamentous ascomycete was from C. parasitica (Zhang et al, 1998), and three pheromone precursor genes, Mf1/1, Mf2/1 and Mf2/2, have been reported to exist in both C. parasitica mating types (Zhang et al, 1998;Turina et al, 2003). The expression of the pheromone precursor genes is mating-type-specific; for example, Mf1/1 is expressed only by Mat-1 strains, and the genes display transcriptional suppression, characteristic of a virus-containing hypovirulent strain.…”

“…The downregulation of pheromone gene expression owing to alterations in the expression of CpMK2, which belongs to a family of pheromone-responsive MAPKs, could imply that pheromone gene expression is dependent on the MAPK pathway, as in S. cerevisiae, and that a MAPK cascade similar to the S. cerevisiae FUS3/KSS1 MAPK pathway exists in C. parasitica. Based on the significant similarities in the structures and expression patterns of the pheromone precursor genes of C. parasitica and the ascomycetous yeasts, it is expected that the features of the mating system of yeasts are conserved in C. parasitica and that pheromone expression in C. parasitica is regulated by the mating-type locus, which encodes transcriptional regulators (Zhang et al, 1998;Shen et al, 1999). Therefore, the finding that disruption of CpMK2 results in inhibited pheromone gene expression could provide clues to intriguing connections between possible mating signals and a MAPK pathway in a phytopathogenic fungus.…”