ICTV Virus Taxonomy Profile: Marnaviridae 2021

Abstract

The family Marnaviridae comprises small non-enveloped viruses with positive-sense RNA genomes of 8.6–9.6 kb. Isolates infect marine single-celled eukaryotes (protists) that come from diverse lineages. Some members are known from metagenomic studies of ocean virioplankton, with additional unclassified viruses described from metagenomic datasets derived from marine and freshwater environments. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Marnaviridae, which is available at ictv.global/report/marnaviridae.

Virion

Structurally characterized members have polyhedral virions, 22–35 nm in diameter, without envelopes or discernible projections (Table 1, Fig. 1). There are four conserved structural proteins, VP1–4. Capsid protein and virion structures have been resolved by cryo-electron microscopy for Chaetoceros tenuissimus RNA virus type II (species Chaetenuissarnavirus II) [1].

Table 1.

Characteristics of members of the family Marnaviridae

Example:	Heterosigma akashiwo RNA virus (AY337486), species Heterosigma akashiwo RNA virus, genus Marnavirus
Virion	Non-enveloped, 22–35 nm with four structural proteins
Genome	8.6–9.6 kb of positive-sense, non-segmented RNA
Replication	Cytoplasmic, involving RNA-directed RNA polymerase and helicase; cytolytic
Translation	Directly from genomic RNA containing one or more internal ribosomal entry sites
Host range	Single-celled eukaryotes (protists) from marine environments
Taxonomy	Realm Riboviria, kingdom Orthornavirae, phylum Pisuviricota, class Pisoniviricetes, order Picornavirales; >5 genera and >15 species

Fig. 1.

Marnaviridae virion structure. (a) Negatively stained transmission electron micrograph of Heterosigma akashiwo RNA virus virions (scale bar=50 nm). (b) Cryo-electron microscopy reconstructed capsid structure at 3.1 Å resolution of Chaetoceros tenuissimus RNA virus type II (species Chaetenuissarnavirus II, genus Sogarnavirus) (PDB structure 6SHL viewed in NGL-WebGL). The three major capsid proteins are represented in dark blue (VP1), light blue (VP2) and yellow (VP3).

Genome

Members possess monopartite positive-sense RNA genomes of 8.6–9.6 kb [2] containing one (Locarnavirus, Marnavirus) or two ORFs (Locarnavirus, Kusarnavirus, Bacillarnavirus, Salisharnavirus, Sogarnavirus); members of the genus Labyrnavirus have a third small overlapping open reading frame (ORF) at the 3′-end of the genome (Fig. 2). Virus genomes encode conserved helicase, RNA-directed RNA polymerase and structural protein domains. Some genomes also encode regions that resemble the 3C cysteine proteinases of members of the family Picornaviridae. Predicted secondary structures within the 5′-non-coding regions and intergenic regions suggest the presence of internal ribosome entry sites. A 3′-poly(A) tail terminates the genome of those viruses that have been isolated.

Fig. 2.

Genome architectures of viruses in the family Marnaviridae. IRES, internal ribosome entry site. RdRP, RNA-directed RNA polymerase. There is a putative IRES in the intergenic region (IGR) of the di-cistronic genomes.

Replication

Viral proteins are synthesized as part of one or two polyproteins. Regardless of whether the genomes are mono- or di-cistronic, the non-structural proteins are encoded in the 5′-region and the structural proteins are encoded in the 3′-region. Replication is cytolytic and various cytopathic effects have been observed [3–8]. Membrane vesicle structures associated with the endoplasmic reticulum [3] are likely to represent sites of RNA synthesis.

Taxonomy

Current taxonomy: www.ictv.global/taxonomy. The genus Marnavirus includes the species Heterosigma akashiwo RNA virus, whose only member has a mono-cistronic genome and infects a harmful algal bloom-forming raphidophyte. Labyrnavirus includes the species Aurantiochytrium single-stranded RNA virus 01, whose only member has a di-cistronic genome and a third small overlapping ORF, which is transcribed as a sub-genomic RNA during infection but may not be functional [9]. The host is Aurantiochytrium sp., a marine thraustochytrid. Locarnavirus includes the species Jericarnavirus B, Sanfarnavirus 1, Sanfarnavirus 2 and Sanfarnavirus 3, members of which were discovered through metagenomic analysis of marine virioplankton. Both mono- and di-cistronic genome organizations are found. Kusarnavirus includes the species Astarnavirus, whose only member infects the diatom Asterionellopsis glacialis and has a di-cistronic genome. Bacillarnavirus includes the species Rhizosolenia setigera RNA virus 01, Chaetoceros tenuissimus RNA virus 01 and Chaetoceros socialis forma radians RNA virus 1, members of which infect diatoms and have di-cistronic genomes. Salisharnavirus includes the species Britarnavirus 1, Britarnavirus 4, Palmarnavirus 128 and Palmarnavirus 473, members of which were discovered by metagenomics and have di-cistronic genomes. Sogarnavirus includes the species Chaetenuissarnavirus II, Chaetarnavirus 2, Britarnavirus 2, Britarnavirus 3, Palmarnavirus 156 and Jericarnavirus A. Members have di-cistronic genomes and have been isolated from diatoms or discovered by metagenomics.

Resources

Full ICTV Report on the family Marnaviridae: https://www.ictv.global/report/marnaviridae