Capsid Structure of dsRNA Fungal Viruses

Most fungal, double-stranded (ds) RNA viruses lack an extracellular life cycle stage and are transmitted by cytoplasmic interchange. dsRNA mycovirus capsids are based on a 120-subunit T = 1 capsid, with a dimer as the asymmetric unit. These capsids, which remain structurally undisturbed throughout the viral cycle, nevertheless, are dynamic particles involved in the organization of the viral genome and the viral polymerase necessary for RNA synthesis. The atomic structure of the T = 1 capsids of four mycoviruses was resolved: the L-A virus of Saccharomyces cerevisiae (ScV-L-A), Penicillium chrysogenum virus (PcV), Penicillium stoloniferum virus F (PsV-F), and Rosellinia necatrix quadrivirus 1 (RnQV1). These capsids show structural variations of the same framework, with 60 asymmetric or symmetric homodimers for ScV-L-A and PsV-F, respectively, monomers with a duplicated similar domain for PcV, and heterodimers of two different proteins for RnQV1. Mycovirus capsid proteins (CP) share a conserved α-helical domain, although the latter may carry different peptides inserted at preferential hotspots. Insertions in the CP outer surface are likely associated with enzymatic activities. Within the capsid, fungal dsRNA viruses show a low degree of genome compaction compared to reoviruses, and contain one to two copies of the RNA-polymerase complex per virion.

Mycoviruses of Penicillium stoloniferum: influence of carbon–nitrogen nutrition upon replication

Carbon–nitrogen ratio experiments indicate that limiting nutrition not only hinders Penicillium stoloniferum host proliferation but reduces total PsV-F and PsV-S virus replication. Results of C-N experiments show a pH-induced autolysis and virus release at minimal C levels. Maximal PsV-F levels and biomass were obtained with glucose and sucrose as C sources. Oleic acid also yielded high biomass and PsV-F yields. Yeast extract was an excellent N source; 2.83 g dry weight biomass and 87 A260 units PsV-F after 96 h of growth. Other nitrogen sources, including amino acids, supported only minimal growth and virus replication. The autolysis phenomenon is pH, not viral-induced. High C and N will support maximal growth and unrestricted virus replication with no cellular lysis. Under low C growth conditions, the replication of PsV-S is favored coupled with high pH and autolysis.