Identification and Complete Genome Sequence of an Alternavirus From Airborne Spores of the Phytopathogenic Fungus Fusarium Bullatum

A double-stranded RNA (dsRNA) mycovirus was isolated from airborne spores of Fusarium bullatum and was named Fusarium bullatum alternavirus 1 (FbAV1). Sequencing analysis and the rapid amplification of cDNA ends (RACE) of 5’ and 3’-end confirmed three segments: dsRNA1 (3546 nt), dsRNA2 (2511 nt) and dsRNA3 (2484 nt). BLASTN search of sequences showed that FbAV1 has 92-96% identity with Fusarium incarnatum Alternavirus 1 (FiAV1). Phylogenetic analysis of the RdRp amino acid sequences suggested that the dsRNA mycovirus in this study clustered with the newly proposed family “Alternaviridae”. This is the first report of FbAV1 mycovirus from airborne spores of a fungus F. bullatum.

Molecular Characterization of a Novel Alternavirus Infecting the Entomopathogenic Fungus Cordyceps Chanhua

In this study, a novel double-stranded RNA (dsRNA) mycovirus, named Cordyceps chanhua alternavirus 1 (CcAV1), was detected in the entomogenous fungus Cordyceps chanhua from China. The complete genome of CcAV1 contained three dsRNA genome segments, dsRNA 1 (3,512 bp), dsRNA 2 (2,655 bp), and dsRNA 3 (2,415 bp). All the three dsRNAs possess a single open reading frame (ORF). DsRNA 1 with 3,512 bp long encoded a putative RNA-dependent RNA polymerase (RdRp), while dsRNA 2 with 2,655 bp long and dsRNA 3 with 2,415 bp long encoded a hypothetical protein 1 (HP 1) and a hypothetical protein 2 (HP 2), respectively. The RdRp, HP 1 and HP 2 sequences had the highest identity of 66.99%, 49.30% and 56.91%, respectively, to those of Aspergillus foetidus dsRNA mycovirus. A maximum-likelihood phylogenetic tree from RdRp sequence revealed that CcAV1 was placed in the clade of the proposed family “Alternaviridae”. Hence, we proposed that Cordyceps chanhua alternavirus 1 is a novel member of the proposed “Alternaviridae”.

Complete Genome Sequence of the First Chrysovirus From the Phytopathogenic Fungus Alternaria Solani on Potato in China

The full genome of a double-stranded RNA (dsRNA) mycovirus, which was isolated from Alternaria solani strain DT-10 causing potato foliar disease and designated as Alternaria solani chrysovirus 1 (AsCV1), consisted of four dsRNA segments (dsRNA 1–4) with the length of 3600 bp, 3128 bp, 2996 bp, and 2714 bp, respectively. RNA-dependent RNA polymerase (RdRp, 1084 amino acids (aa)), putative capsid protein (905 aa), Alphachryso-P3 (835 aa), and Alphachryso-P4 (729 aa) were encoded by dsRNA1, dsRNA2, dsRNA3, and dsRNA4, respectively, which had the highest identities of 41.77%-72.38% to the counterparts of Helminthosporium victoriae virus 145S (HvV145S) in the genus Alphachrysovirus of the family Chrysoviridae. Moreover, the 5′-untranslated regions (UTRs) with several unique inserts (3–37 bp) and deletions (5–64 bp) of AsCV1 dsRNA 1–4 shared 51.65%-68.01% identities to those of HvV145S. Phylogenetic analysis of RdRp suggested that AsCV1 clustered the most closely with HvV145S. Based on the characteristics of distinct host, low identities of encoded proteins, special traits in 5′-UTRs of dsRNA 1–4, and phylogenetic analysis of RdRp, AsCV1 was confirmed to be a new species in the genus Alphachrysovirus. To our best knowledge, this is the first alphachrysovirus identified from phytopathogenic A. solani.

The Whole Genome Sequence of a Novel Chrysovirus From Beauveria Bassiana Vuillemin, an Entomopathogenic Fungus

Beauveria bassiana, an entomopathogenic fungus, has a wide host range and is used for arthropod pest control worldwide. Here, we report the discovery and characterization of a novel double-stranded RNA (dsRNA) mycovirus Beauveria bassiana chrysovirus 2 (BbCV2), isolated from the B. bassiana from China. The genome of the virus was determined by metagenomic sequencing, RT-PCR, and RACE cloning comprises four dsRNA segments that are 3441bp, 2779bp, 2925bp, and 2688bp long, respectively, each of them contains a single ORF, the first one (ORF1) encoding a 1115-amino-acid-long protein (122.65 kDa) with a conserved RNA-dependent RNA polymerase (RdRp) motif, its sequences showed the highest identity of only 16.13% to that of the Beauveria bassiana chrysovirus 1. The second ORF (ORF-2) encoding a 807-amino-acid-long coat protein (CP) (88.77 kDa). The virus constitutes a new member of the chrysoviridea family from B. bassiana.

Meiotic viral attenuation through an ancestral apoptotic pathway

The programmed release of apoptogenic proteins from mitochondria is a core event of apoptosis, though ancestral roles of this phenomenon are not known. In mammals, one such apoptogenic protein is Endonuclease G (EndoG), a conserved nuclease that fragments the DNA of dying cells. In this work, we show that budding yeast executes meiotically programmed mitochondrial release of an EndoG homologue, Nuc1, during sporulation. In contrast to EndoG’s ostensible pro-death function during apoptosis, Nuc1 mitochondrial release attenuates the cytosolic dsRNA mycovirus, Killer, protecting spores from a lethal accumulation of its encoded toxin. Our identification of cell-protective viral attenuation as a target of this rudimentary apoptotic pathway illuminates a primordial role for mitochondrial release of EndoG.One Sentence SummaryYeast sporulation induces release of mitochondrial endonuclease G to accomplish viral attenuation.

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.