Reduced genome of the thioautotrophic intracellular symbiont in a deep-sea clam, Calyptogena okutanii.
Although dense animal communities at hydrothermal vents and cold seeps rely on symbioses with chemoautotrophic bacteria [1, 2], knowledge of the mechanisms underlying these chemosynthetic symbioses is still fragmentary because of the difficulty in culturing the symbionts and the hosts in the laboratory. Deep-sea Calyptogena clams harbor thioautotrophic bacterial symbionts in their gill epithelial cells [1, 2]. They have vestigial digestive tracts and nutritionally depend on their symbionts [3], which are vertically transmitted via eggs [4]. To clarify the symbionts' metabolic roles in the symbiosis and adaptations to intracellular conditions, we present the complete genome sequence of the symbiont of Calyptogena okutanii. The genome is a circular chromosome of 1,022,154 bp with 31.6% guanine + cytosine (G + C) content, and is the smallest reported genome in autotrophic bacteria. It encodes 939 protein-coding genes, including those for thioautotrophy and for the syntheses of almost all amino acids and various cofactors. However, transporters for these substances to the host cell are apparently absent. Genes that are unnecessary for an intracellular lifestyle, as well as some essential genes (e.g., ftsZ for cytokinesis), appear to have been lost from the symbiont genome. Reductive evolution of the genome might be ongoing in the vertically transmitted Calyptogena symbionts.
Kuwahara
Hirokazu
H
Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Yokosuka 237-0061, Japan.
Yoshida
Takao
T
Takaki
Yoshihiro
Y
Shimamura
Shigeru
S
Nishi
Shinro
S
Harada
Maiko
M
Matsuyama
Kazuyo
K
Takishita
Kiyotaka
K
Kawato
Masaru
M
Uematsu
Katsuyuki
K
Fujiwara
Yoshihiro
Y
Sato
Takako
T
Kato
Chiaki
C
Kitagawa
Masanari
M
Kato
Ikunoshin
I
Maruyama
Tadashi
T
D000818:Animals; D052818:Autotrophic Processes; D001419:Bacteria / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D001692:Biological Transport; D049872:Bivalvia / Q000382:microbiology*; D002244:Carbon / Q000378:metabolism; D004734:Energy Metabolism; D016680:Genome, Bacterial; D016013:Likelihood Functions; D013455:Sulfur / Q000378:metabolism; D013559:Symbiosis
D013455:Sulfur; D002244:Carbon