A special satellite-like dsRNA of a novel hypovirus from Pestalotiopsis fici broadens the definition of 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 a newly proposed 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 is 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 dsRNAs 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 dsRNA3, 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 satellite. IMPORTANCESatellites in association with plant or animal viruses have been largely detected and characterized, while those from mycoviruses together with their roles remain far less determined. Here, a special satellite-like dsRNA (SatL-dsRNA) together with its helper virus, a novel hypovirus from Pestalotiopsis fici, was identified and characterized. This SatL-dsRNA lacks a substantial open reading frame and a poly (A) tail, which is unlike the known satellite dsRNAs of hypoviruses. It is also unlike those in association with Totiviridae and Partitiviridae since the latters are encapsidated in coat proteins. As up-regulated expression of the SatL-dsRNA, the helper virus genome was significantly down-regulated, suggesting that it negatively regulates the genomic expression of the helper virus. This special SatL-dsRNA has substantial sequence homology with the host viral genome and is not encapsidated in the coat protein of its helper virus, which represents a novel class of satellite-like nucleic acids, and it broadens the definition of satellite.