MutS proteins are ubiquitous in cellular organisms and have important roles in DNA mismatch repair or recombination. In the virus world, the amoeba-infecting Mimivirus, as well as the recently sequenced Cafeteria roenbergensis virus are known to encode a MutS related to the homologs found in octocorals and e-proteobacteria. To explore the presence of MutS proteins in other viral genomes, we performed a genomic survey of four giant viruses ('giruses') (Pyramimonas orientalis virus (PoV), Phaeocystis pouchetii virus (PpV), Chrysochromulina ericina virus (CeV) and Heterocapsa circularisquama DNA virus (HcDNAV)) that infect unicellular marine algae. Our analysis revealed the presence of a close homolog of Mimivirus MutS in all the analyzed giruses. These viral homologs possess a specific domain structure, including a C-terminal HNH-endonuclease domain, defining the new MutS7 subfamily. We confirmed the presence of conserved mismatch recognition residues in all members of the MutS7 subfamily, suggesting their role in DNA mismatch repair rather than DNA recombination. PoV and PpV were found to contain an additional type of MutS, which we propose to call MutS8. The MutS8 proteins in PoV and PpV were found to be closely related to homologs from 'Candidatus Amoebophilus asiaticus', an obligate intracellular amoeba-symbiont belonging to the Bacteroidetes. Furthermore, our analysis revealed that MutS7 and MutS8 are abundant in marine microbial metagenomes and that a vast majority of these environmental sequences are likely of girus origin. Giruses thus seem to represent a major source of the underexplored diversity of the MutS family in the microbial world.

Heterocapsa circularisquama DNA virus (HcDNAV; previously designated as HcV) is a giant virus (girus) with a ~356-kbp double-stranded DNA (dsDNA) genome. HcDNAV lytically infects the bivalve-killing marine dinoflagellate H. circularisquama, and currently represents the sole DNA virus isolated from dinoflagellates, one of the most abundant protists in marine ecosystems. Its morphological features, genome type, and host range previously suggested that HcDNAV might be a member of the family Phycodnaviridae of Nucleo-Cytoplasmic Large DNA Viruses (NCLDVs), though no supporting sequence data was available. NCLDVs currently include two families found in aquatic environments (Phycodnaviridae, Mimiviridae), one mostly infecting terrestrial animals (Poxviridae), another isolated from fish, amphibians and insects (Iridoviridae), and the last one (Asfarviridae) exclusively represented by the animal pathogen African swine fever virus (ASFV), the agent of a fatal hemorrhagic disease in domestic swine. In this study, we determined the complete sequence of the type B DNA polymerase (PolB) gene of HcDNAV. The viral PolB was transcribed at least from 6 h post inoculation (hpi), suggesting its crucial function for viral replication. Most unexpectedly, the HcDNAV PolB sequence was found to be closely related to the PolB sequence of ASFV. In addition, the amino acid sequence of HcDNAV PolB showed a rare amino acid substitution within a motif containing highly conserved motif: YSDTDS was found in HcDNAV PolB instead of YGDTDS in most dsDNA viruses. Together with the previous observation of ASFV-like sequences in the Sorcerer II Global Ocean Sampling metagenomic datasets, our results further reinforce the ideas that the terrestrial ASFV has its evolutionary origin in marine environments.

Since the discovery of a single-stranded RNA virus infectious to Rhizosolenia setigera (Bacillariophyceae), several novel diatom-infecting viruses have been isolated and examined as a potential mortality source for diatom populations. Here, we report the isolation and characterisation of a new single-stranded DNA (ssDNA) virus (CtenDNAV) that causes lysis of the cosmopolitan diatom species Chaetoceros tenuissimus (Meunier). The virion is 37 nm in diameter and accumulates in the nucleus of host cells. CtenDNAV has a closed circular ssDNA genome (5639 nt), which includes a partially double-stranded region (875 bp) and at least 3 major open reading frames (ORFs). One ORF is similar to putative replicase-related proteins of the previously reported ssDNA diatom viruses, CsalDNAV and CdebDNAV. On the basis of the genome structure and host range, CtenDNAV is considered to belong to the new genus Bacilladnavirus. CtenDNAV is the second C. tenuissimus virus; the first was the single-stranded RNA virus CtenRNAV. Data from the present study suggest that the C. tenuissimus natural population is affected by at least 2 viruses differing in genome type.KEY WORDS: Bacilladnavirus · Chaetoceros · Diatom · ssDNA · Virus · Rod-shaped particle Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 64: [175][176][177][178][179][180][181][182][183][184] 2011 Bacilladnavirus. This is the first report of 2 viruses with different genome types (ssRNA and ssDNA) that share the same diatom host. MATERIALS AND METHODSAlgal cultures and growth conditions. The axenic clonal algal strains used for virus isolation in the present study were Chaetoceros tenuissimus (Meunier) Strains 2-10 ( Shirai et al. 2008), 2-6 and AG07-C03. Strain 2-10, previously used for isolation of CtenRNAV, was isolated from Maiko, Harima-Nada, Seto Inland Sea, Japan, on 10 August 2002 . The 2-6 and AG07-C03 strains were isolated from surface water in Hiroshima Bay, Japan, on 9 July 2002 and surface water in Ago Bay, Japan, on 4 July 2007, respectively. Algal cultures were grown in modified SWM3 medium enriched with 2 nM Na 2 SeO 3 (Chen et al. 1969, Itoh & Imai 1987 under a 12 h light:12 h dark cycle of ca. 110 to 150 µmol photons m -2 s -1 using cool white fluorescent illumination at 15°C.Virus isolation. Water samples (0 m and 0.2 m above the bottom [B-0.2 m]) and sediment samples (0 to 3 cm depth for Ariake Sound and 0 to 1 cm depth for Hiroshima Bay) were collected from the mouth of the Shiotsuka River in Ariake Sound and from Itsukaichi Fishing Port in Hiroshima Bay, Japan, from 26 April 2005 through 15 October 2007. The water samples were filtered through 0.2 µm Dismic-25cs filters (Advantec) to remove eukaryotic microorganisms and most bacteria. The sediment sample (12 g) was shaken with 12 ml of medium (400 rpm at room temperature for 30 min) and centrifuged at 860 × g at 4°C for 10 min. The supernatants were sequentially passed through GF/F filters (Whatman) and 0.2 µm Dismic-25cs filters (Advantec). Aliquo...

Diatoms are prominent primary producers that play an important role in global carbon cycles. Diatom dynamics are, therefore, important for biogeochemistry, fisheries and earth science. Viral infection is now assumed to be one of the most significant factors affecting diatom dynamics. However, few studies, based on practical field data, have focused on the relationship between diatoms and viruses in natural waters. To elucidate on this relationship, we assessed the temporal change in the abundance of diatoms and their viruses on the western coast of Japan from 2004 to 2009. Three species of diatoms were used as hosts to enumerate viruses in water and sediment samples: Chaetoceros debilis, C. tenuissimus and C. salsugineum. Diatom viruses in the water column rapidly increased during C. debilis and C. tenuissimus blooms, maintaining a high abundance throughout the blooming period. These data suggest the potential importance of viruses in controlling the population dynamics of diatoms in natural environments. The number of viruses in sediments fluctuated considerably, even during non-blooming periods, which suggests that virus-infected diatom cells were supplied to the sediments from water-column populations throughout the year. This process might be an important strategy to reduce the impact of viruses on diatom populations.