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    STUDIA BIOLOGIA - Issue no. 1 / 2019  

  Abstract:  Relatively few haloviruses have been isolated and their genomes sequenced, even though they are present at high levels in hypersaline waters, and are likely to be major drivers of host species evolution. We recently published the genome sequences of two haloviruses, phiH1 and ChaoS9 (Dyall-Smith et al., 2018, 2019), both myoviruses infecting Halobacterium salinarum, and both isolated following lysis events of laboratory cultures used to produce bacteriorhodopsin. While phiH caused a minor loss of production, ChaoS9 was much worse, lysing a 1 m3 culture. The two viral genomes are remarkably similar, even though they originated from independent lysis events many years apart. Both genomes were also similar to halovirus phiCh1 (host: Natrialba magadii), and while the three show many patchy differences (mosaicism), they form a distinct clade by various methods of phylogenetic tree reconstruction. Surprisingly, the major capsid protein (MCP) of ChaoS9 is not similar to those of phiH1 or phiCh1, but is closely related to the MCP of HHTV-1 (host: Haloarcula hispanica), a siphovirus that is otherwise unrelated. In addition, a number of previously unexamined halovirus isolates, kept frozen as clarified cell lysates for many years, have been analysed by high-throughput sequencing of total DNA. This revealed the presence not only of novel haloviruses but also proviruses derived from the host cell genomes. The surprising presence of induced proviruses may be common in haloarchaea, and could potentially threaten experimental studies or large scale technical applications involving two or more species.

Keywords: haloarchaea, halobacteria, halovirus, hypersaline, provirus.
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