A special satellite-like RNA of a novel hypovirus from Pestalotiopsis fici broadens the definition of fungal satellite

Abstract

Satellites associated with plant or animal viruses have been largely detected and characterized, while those from mycoviruses together with their roles remain far less determined. Three dsRNA segments (dsRNA 1 to 3 termed according to their decreasing sizes) were identified in a strain of phytopathogenic fungus Pestalotiopsis fici AH1-1 isolated from a tea leaf. The complete sequences of dsRNAs 1 to 3, with the sizes of 10316, 5511, and 631 bp, were determined by random cloning together with a RACE protocol. Sequence analyses support that dsRNA1 is a genome of a novel hypovirus belonging to genus Alphahypovirus of the family Hypoviridae, tentatively named Pestalotiopsis fici hypovirus 1 (PfHV1); dsRNA2 is a defective RNA (D-RNA) generating from dsRNA1 with septal deletions; and dsRNA3 is the satellite component of PfHV1 since it could be co-precipitated with other dsRNA components in the same sucrose fraction by ultra-centrifuge, suggesting that it is encapsulated together with PfHV1 genomic dsRNAs. Moreover, dsRNA3 shares an identical stretch (170 bp) with dsRNAs 1 and 2 at their 5′ termini and the remaining are heterogenous, which is distinct from a typical satellite that generally has very little or no sequence similarity with helper viruses. More importantly, dsRNA3 lacks a substantial open reading frame (ORF) and a poly (A) tail, which is unlike the known satellite RNAs of hypoviruses, as well as unlike those in association with Totiviridae and Partitiviridae since the latters are encapsidated in coat proteins. As up-regulated expression of RNA3, dsRNA1 was significantly down-regulated, suggesting that dsRNA3 negatively regulates the expression of dsRNA1, whereas dsRNAs 1 to 3 have no obvious impact on the biological traits of the host fungus including morphologies and virulence. This study indicates that PfHV1 dsRNA3 is a special type of satellite-like nucleic acid that has substantial sequence homology with the host viral genome without encapsidation in a coat protein, which broadens the definition of fungal satellite.

Identification and Characterization of a Novel Hypovirus from the Phytopathogenic Fungus Botryosphaeria dothidea

“…Notably, the effects of hypovirus infection on their host varied. For example, CHV4/SR2, FgHV1, VcHV1, and Pestalotiopsis fici hypovirus 1 (PfHV1) had no significant effect on their hosts [41,43,66,67]. By contrast, CHV1/EP713, CHV2/NB58, CHV3/GH2, and FgHV2 were associated with the hypovirulence of their host [42,[68][69][70][71].…”

“…However, the infection of BcHV1 alone without the satellite RNA significantly reduced virulence without affecting mycelial growth [46]. A recent study showed that satellite-like RNA could negatively regulate the expression of helper virus PfHV1 [66]. Notably, BdHV1/SXD111 had no evident impact on the morphologies and virulence of the host fungus.…”

The non-template functions of helper virus RNAs create optimal replication conditions to enhance the proliferation of satellite RNAs

“…Except for a limited number of mycovirus-associated satRNAs [15][16][17], the majority of satRNAs are associated with plant-infecting positive sense (+) RNA viruses [14], which represent the largest category of plant viruses and pose significant threats to global agriculture. Upon infection, these (+) RNA viruses release their genomic RNAs to produce viral replication proteins.…”