Diversity and Current Classification of dsRNA Bacteriophages

Abstract

Half a century has passed since the discovery of Pseudomonas phage phi6, the first enveloped dsRNA bacteriophage to be isolated. It remained the sole known dsRNA phage for a quarter of a century and the only recognised member of the Cystoviridae family until the year 2018. After the initial discovery of phi6, additional dsRNA phages have been isolated from globally distant locations and identified in metatranscriptomic datasets, suggesting that this virus type is more ubiquitous in nature than previously acknowledged. Most identified dsRNA phages infect Pseudomonas strains and utilise either pilus or lipopolysaccharide components of the host as the primary receptor. In addition to the receptor-mediated strictly lytic lifestyle, an alternative persistent infection strategy has been described for some dsRNA phages. To date, complete genome sequences of fourteen dsRNA phage isolates are available. Despite the high sequence diversity, similar sets of genes can typically be found in the genomes of dsRNA phages, suggesting shared evolutionary trajectories. This review provides a brief overview of the recognised members of the Cystoviridae virus family and related dsRNA phage isolates, outlines the current classification of dsRNA phages, and discusses their relationships with eukaryotic RNA viruses.

Keywords: bacteriophage; cystovirus; dsRNA virus; virus taxonomy.

Figure 1

Schematic presentation of phi6 virion. The size of the enveloped virion is about 85 nm [27]. The lipid-protein envelope encloses the nucleocapsid, which comprises two concentric protein shells: the nucleocapsid surface shell and the polymerase complex. Hexamers of the packaging NTPase P4 are attached on the icosahedral five-fold vertices of the polymerase complex and protrude through the nucleocapsid surface shell [28]. The spooled dsRNA genome is tightly packed inside the polymerase complex [29] together with about ten copies of the viral polymerase subunit P2 [30].

Figure 2

Taxonomy of eukaryotic and bacterial dsRNA viruses currently recognised by the International Committee on Taxonomy of Viruses. Two of the five phyla under the kingdom Orthornavirae are depicted. These two phyla, Pisuviricota and Duplornaviricota, comprise all the current dsRNA virus families, except Birnaviridae, which has not been assigned to any phylum. All the three classes of Duplornaviricota (Vidaverviricetes, Resentoviricetes and Chrymotiviricetes) comprise solely dsRNA viruses, while Pisuviricota contains both ssRNA (two classes; grey boxes) and dsRNA viruses (class Duplopiviricetes). dsRNA virus taxa are in bold, dsRNA phage taxa are underlined and in blue boxes. ssRNA virus families under Duplopiviricetes are in grey font.