The moniker “model organism” is given to certain species if they have features that make them particularly useful for discovery of biological and biomedical principles. The features that make an organism particularly valuable as a model include ease of growth, availability of experimental tools (e.g., genetic manipulation), evolutionary relationship to other organisms of interest (e.g., human, crops, pathogens, etc.), and presence of interesting phenotypes.
One limitation in the use of model organisms is that they are very sparsely distributed across the tree of life. This is particularly true for the archaea 
. Though this group represents one of the three main lines of descent in the tree of life, only a few species have been developed into what could be considered true model organisms. The limited number of model systems has presented a challenge for characterizing the biology of this key group of organisms. There are a number of reasons for this, not the least of which is the difficulty in growing many archaea.
However, there is one group of archaea for which many of the species are relatively easy to work with - the Halobacteriaceae. Though these species are obligate halophiles requiring high salt conditions to grow, because they are aerobic and mesophilic (they grow at moderate temperature), they can be grown in conditions much like those used for other model organisms such as the bacterium Escherichia coli and the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This ease of growth also makes Halobacteriaceae useful for introducing students to working with archaea since growing many other archaea requires expensive equipment or extensive training due to their thermophilic and/or anaerobic growth requirements.
form a monophyletic group within the phylum Euryarchaea of the domain Archaea. The Halobacteriaceae
include 26 named genera (each with at least one cultured species - see http://www.the-icsp.org/taxa/halobacterlist.htm 
(abbreviated as Hbt
.), Haloadaptus, Halalkalicoccus, Haloarcula
.), Halobaculum, Halobiforma, Halococcus
.), Halogeometricum, Halomicrobium, Halopiger, Haloplanus, Haloquadratum
), Halorhabdus, Halorubrum, Halosimplex, Halostagnicola, Haloterrigena, Halovivax, Natrialba, Natrinema, Natronobacterium
.), and Natronorubrum
. As with most other groups of bacteria and archaea, many lineages are only known through rRNA-based studies of uncultured organisms (e.g., see 
Of the haloarchaea, two very distinct species have been developed into models for experimental studies: the extreme halophile Halobacterium
sp. NRC-1 (optimal NaCl 2.5–4 M) and the moderate halophile Hfx. volcanii
(optimal NaCl 1.7–2.5 M). We note here that Halobacterium
sp. NRC-1 and Halobacterium salinarum
) refer to two very closely related isolates. Next to the different salt requirements, Hbt.
sp. NRC-1 has some biological properties not found in Hfx. volcanii
such as phototrophic growth employing rhodopsin-like proteins and the formation of gas vesicles. Conversely, Hfx. volcanii
can degrade sugars such as glucose and synthesizes most of its amino acids. This not only allows for the study of archaeal carbohydrate utilization and amino acid biosynthesis, but also has proven to be highly useful for genetic selections etc. as Hfx. volcanii
grows well on defined media. This is in contrast to Hbt.
sp. NRC-1, which cannot degrade sugars and only synthesizes a minor subset of its amino acids leading, at least in part, to its rather poor growth on defined media. Hence both of these haloarchaea are highly valuable models to understand the diversity and ecology of high-salt environments but also to learn from their similarities and differences. Interestingly, both of these species of haloarchaea are highly polyploid 
While molecular biological and biochemical tools have been developed for both of these haloarchaea, the requirement for salinity close to saturation and the lack of a well-defined growth medium can interfere with Hbt.
sp. NRC-1 in vivo
assays. Moreover its highly mobile insertion elements cause frequent mutations 
. In contrast, Hfx. volcanii
grows on simple defined minimal media (either solid or liquid), accepts a wider range of relatively lower salt concentrations than most other extreme halophiles (including Hbt.
sp. NRC-1) and its genome is significantly more stable 
Hence, over the past two decades this biochemically and genetically tractable moderate haloarchaeon has been invaluable in revealing insight into archaeal biology ranging from transcription to protein transport, modification and degradation. These studies have taken advantage of a diverse set of genetic, molecular and biochemical tools including among others, a simple knockout strategy 
, inducible promoters 
and protein purification protocols, efficient, straight-forward transformation methods 
, shuttle vectors 
, a diversity of selectable markers 
, beta-galactosidase 
and short lived green fluorescent protein reporters 
, an ordered cosmid library 
and genetic and physical maps 
These tools have helped enable the use of Hfx. volcanii
as a model for studies of various archaeal cellular processes such as protein transport 
, protein glycosylation 
, lipid modification 
, tRNA processing 
, gas vesicle formation 
, nucleotide synthesis 
, transcription 
, protein degradation 
, DNA repair and recombination 
and DNA replication 
Here we report on the sequencing of the genome of the type strain DS2. This strain was first described in 1975 
following its isolation from bottom sediment of the Dead Sea. It was initially known as Halobacterium volcanii
(in reference to Benjamin Elazari Volcani who first demonstrated the existence of indigenous microbial communities in high salt environments 
). We focused on the type strain to serve as a reference point for this species 
, and it is worth noting that strains WFD11 
and DS70 
are derived from DS2 and are widely used in the haloarchaeal community. This genome sequence, with proteome 
and transcriptome 
analyses in place, has been the missing piece in making this organism an outstanding model. Here we present analysis of this genome sequence in the context of previously obtained in vivo
and in vitro
work as well as the comparison of this sequence to four other haloarchaeal genomes.
We note that the genome sequence of this organism was made available a few years ago to the community in order to accelerate research and work on this organism. Using the genome data many new findings have been reported including but not limited to studies of the agl
gene cluster 
, delineation of 3′ and 5′ UTRs 
, characterization of small RNAs 
, chromosomal replication 
, RNA modification genes 
, and shotgun proteomics 
. While these studies have been enabled by the genome data, the lack of a publication describing the sequencing and analysis has been a hindrance. Thus in this paper we describe the sequencing and initial analyses of the genome.