Halovirus PH1 infects Haloarcula hispanica and was isolated from an Australian salt lake. The burst size in single-step growth conditions was 50–100？PFU/cell, but cell density did not decrease until well after the rise (4–6？hr p.i.), indicating that the virus could exit without cell lysis. Virions were round, 51？nm in diameter, displayed a layered capsid structure, and were sensitive to chloroform and lowered salt concentration. The genome is linear dsDNA, 28,064？bp in length, with 337？bp terminal repeats and terminal proteins, and could transfect haloarchaeal species belonging to five different genera. The genome is predicted to carry 49 ORFs, including those for structural proteins, several of which were identified by mass spectroscopy. The close similarity of PH1 to SH1 (74% nucleotide identity) allowed a detailed description and analysis of the differences (divergent regions) between the two genomes, including the detection of repeat-mediated deletions. The relationship of SH1-like and pleolipoviruses to previously described genomic loci of virus and plasmid-related elements (ViPREs) of haloarchaea revealed an extensive level of recombination between the known haloviruses. PH1 is a member of the same virus group as SH1 and HHIV-2, and we propose the name halosphaerovirus to accommodate these viruses. 1. Introduction Viruses of Archaea (archaeoviruses [1]) show considerable diversity and encompass novel morphotypes not seen in Bacteria or Eukarya. Relatively few have been examined in detail, partly because of the demanding growth requirements of many extremophilic Archaea (particularly thermophiles), and also because genetic analysis is often technically difficult compared to bacterial systems such as Escherichia coli. Although viruses of thermophilic Archaea show the most innovative capsids and replication strategies [1], the viruses of halophilic Archaea (haloarchaea) are of increasing attention as new isolates are found with unexpected properties. Many of the earliest reported haloviruses, including all those described before 1998, are bacteriophage-like (Caudovirales) with typical head-tail capsids and linear dsDNA genomes. These include groups of related viruses, such as the ΦH-like genus (ΦH, ΦCh1, and BJ1) [2–4] and the unassigned virus group comprising of HF1 and HF2 [5]. The first spindle-shaped halovirus, His1, was reported in 1998 [6, 7], and the first round virus, SH1, was described in 2003 [8, 9], and electron microscopic studies indicate that these morphotypes dominate in natural waters [10–12]. Over the last 5 years, there has been a
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