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The Guttaviridae is a family of enveloped viruses infecting hyperthermophilic archaea. The virions are ovoid or droplet-shaped, with a diameter of 55–80 nm and a length of 75–130 nm. The genome is a circular dsDNA molecule of around 14–20 kbp. The family includes two genera, Alphaguttavirus and Betaguttavirus, each with a single species.
Table 1.Guttaviridae. Characteristics of the family Guttaviridae.
Aeropyrum pernix ovoid virus 1 (HE580237), species Aeropyrum pernix ovoid virus 1, genus Betaguttavirus
Enveloped virions of ovoid shape, with a diameter of 55–80 nm and a length of 75–130 nm.
Circular dsDNA of ~14–20 kbp
Genome is likely to be replicated by the host replisome
Hyperthermophilic archaea, phylum Crenarchaeota
Two genera: Alphaguttavirus and Betaguttavirus
The virions of members of the Guttaviridae have an ovoid shape, measuring 55 x 75 nm (for Aeropyrum pernix ovoid virus 1 (APOV1), Betaguttavirus) to 80 x 130 nm (for Sulfolobus newzealandicus droplet-shaped virus (SNDV), Alphaguttavirus), when analysed by cryo-electron microscopy (Arnold et al., 2000, Mochizuki et al., 2011). The virion surface is covered by globular subunits, which are ~3.5 nm in width. In negative-contrast electron micrographs, the virions are slightly pleomorphic, most displaying a droplet-like shape. Virions of SNDV are decorated with dense filaments attached to the pointed end of the virion; such appendages were not observed in the virions of APOV1 (Figure 1.Guttaviridae). The droplet-shaped morphology is unprecedented among viruses of bacteria and eukaryotes, and represents a group of archaea-specific virion morphotypes (Prangishvili et al., 2017).
Figure 1.Guttaviridae. Virions of Aeropyrum pernix ovoid virus 1 (A and B) and Sulfolobus newzealandicus droplet-shaped virus (C). Virions were negatively stained (A, C) and embedded in ice (B). Scale bars, 100 nm. (A and B, reproduced with permission from (Mochizuki et al., 2011); (C, Courtesy of W. Zillig)
Virion densities have not been determined because virions are unstable in CsCl and lyse.
Covalently closed circular dsDNA between 14–20 kbp.
SNDV virions have one major capsid protein of ~17.5 kDa (Arnold et al., 2000), whereas those of APOV1 include one major capsid protein of ~10.5 kDa and at least 2 minor capsid proteins (Mochizuki 2012).
Guttaviruses appear to be enveloped but the exact lipid composition is not known.
There is no sequence information for SNDV. The dsDNA genome of APOV1 is 13,769 bp (56.5% GC) and contains 21 ORFs that could encode proteins of more than 56 amino acids, including an integrase of the tyrosine recombinase superfamily, a DnaA-like ATPase, a glycoside hydrolase and several DNA-binding proteins containing the helix-turn-helix motifs (Figure 2.Guttaviridae). In most cases, the ORFs are preceded by recognizable putative ribosome binding sites. The majority of putative genes (14) are located on one strand of the DNA. Of the seven putative genes on the opposite strand, five are clustered together (Figure 2.Guttaviridae). Two of the putative genes show sequence similarity to genes of members of the Fuselloviridae encoding an integrase and a DnaA-like protein, whereas three other genes, including the one for glycoside hydrolase, are most closely related to those encoded by hyperthermophilic crenarchaea. APOV1 does not carry a gene for a DNA polymerase, suggesting that its genome is replicated by the host replisome. The DnaA-like ATPase of APOV1 is distantly related to the bacterial DNA replication initiator DnaA and, thus, might be involved in the initiation of the APOV1 genome replication. However, experimental evidence to support this prediction is missing.
Figure 2.Guttaviridae. Genome map of Aeropyrum pernix ovoid virus 1. Functionally annotated open reading frames are highlighted with different colours. The names of the genes include the information on the length and function (when available) of the encoded proteins. Abbreviations: (w)HTH, (winged) helix-turn-helix DNA-binding proteins; MCP, major capsid protein.
APOV1 resides in the genome of Aeropyrum pernix as a provirus integrated into the tRNALeu gene (Mochizuki et al., 2011). The attachment site on the viral genome (attP) is located within the integrase gene. Consequently, upon integration of the APOV1 genome into the host chromosome, the integrase gene is partitioned, as has been also described for fuselloviruses. Excision of the proviral APOV1 genome from the host chromosome, followed by genome replication and virion production, is induced under suboptimal growth conditions, namely, reduced aeration (A. pernix is an obligate aerobe). Similarly, SNDV resides within the host cell in a carrier state as an episomal provirus, which is spontaneously induced at the early stationary growth phase. SNDV virion release is associated with host cell lysis (Arnold et al., 2000).
In the absence of the SNDV genome sequence, the Alphaguttavirus and Betaguttavirus genera are distinguished by differences in the detailed morphology of SNDV and APOV1 virions.
Gutta from Latin gutta, "drop".
As only the APOV1 genome sequence is available, there is no analysis of phylogenetic relationships in the family.
The bipartite gene-sharing network analysis of the archaeal virosphere showed that APOV1 forms a common module with fuselloviruses, indicating that the two groups of hyperthermophilic archaeal viruses might be evolutionarily related (Krupovic et al., 2018, Iranzo et al., 2016). Fuselloviruses and APOV1 share genes for an integrase of the tyrosine recombinase superfamily and a DnaA-like ATPase.
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