A genomic analysis of the archaeal system Ignicoccus hospitalis-Nanoarchaeum equitans.
BACKGROUND
The relationship between the hyperthermophiles Ignicoccus hospitalis and Nanoarchaeum equitans is the only known example of a specific association between two species of Archaea. Little is known about the mechanisms that enable this relationship.


RESULTS
We sequenced the complete genome of I. hospitalis and found it to be the smallest among independent, free-living organisms. A comparative genomic reconstruction suggests that the I. hospitalis lineage has lost most of the genes associated with a heterotrophic metabolism that is characteristic of most of the Crenarchaeota. A streamlined genome is also suggested by a low frequency of paralogs and fragmentation of many operons. However, this process appears to be partially balanced by lateral gene transfer from archaeal and bacterial sources.


CONCLUSIONS
A combination of genomic and cellular features suggests highly efficient adaptation to the low energy yield of sulfur-hydrogen respiration and efficient inorganic carbon and nitrogen assimilation. Evidence of lateral gene exchange between N. equitans and I. hospitalis indicates that the relationship has impacted both genomes. This association is the simplest symbiotic system known to date and a unique model for studying mechanisms of interspecific relationships at the genomic and metabolic levels.
Podar
Mircea
M
Biosciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37831, USA. podarm@ornl.gov
Anderson
Iain
I
Makarova
Kira S
KS
Elkins
James G
JG
Ivanova
Natalia
N
Wall
Mark A
MA
Lykidis
Athanasios
A
Mavromatis
Kostantinos
K
Sun
Hui
H
Hudson
Matthew E
ME
Chen
Wenqiong
W
Deciu
Cosmin
C
Hutchison
Don
D
Eads
Jonathan R
JR
Anderson
Abraham
A
Fernandes
Fillipe
F
Szeto
Ernest
E
Lapidus
Alla
A
Kyrpides
Nikos C
NC
Saier
Milton H
MH
Richardson
Paul M
PM
Rachel
Reinhard
R
Huber
Harald
H
Eisen
Jonathan A
JA
Koonin
Eugene V
EV
Keller
Martin
M
Stetter
Karl O
KO
D001692:Biological Transport; D019620:Desulfurococcaceae / Q000235:genetics* / Q000502:physiology; D004734:Energy Metabolism; D022761:Gene Transfer, Horizontal; D020745:Genome, Archaeal; D046588:Nanoarchaeota / Q000235:genetics* / Q000502:physiology; D010802:Phylogeny; D013559:Symbiosis