title: Reduced genome of the thioautotrophic intracellular symbiont in a deep-sea clam, Calyptogena okutanii.
abstract: 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.
authors.affiliation: Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Yokosuka 237-0061, Japan.
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Kuwahara
authors.forename: Takao
authors.initials: T
authors.lastname: Yoshida
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Takaki
authors.forename: Shigeru
authors.initials: S
authors.lastname: Shimamura
authors.forename: Shinro
authors.initials: S
authors.lastname: Nishi
authors.forename: Maiko
authors.initials: M
authors.lastname: Harada
authors.forename: Kazuyo
authors.initials: K
authors.lastname: Matsuyama
authors.forename: Kiyotaka
authors.initials: K
authors.lastname: Takishita
authors.forename: Masaru
authors.initials: M
authors.lastname: Kawato
authors.forename: Katsuyuki
authors.initials: K
authors.lastname: Uematsu
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Fujiwara
authors.forename: Takako
authors.initials: T
authors.lastname: Sato
authors.forename: Chiaki
authors.initials: C
authors.lastname: Kato
authors.forename: Masanari
authors.initials: M
authors.lastname: Kitagawa
authors.forename: Ikunoshin
authors.initials: I
authors.lastname: Kato
authors.forename: Tadashi
authors.initials: T
authors.lastname: Maruyama
mesh_terms: 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
chemical_list: D013455:Sulfur; D002244:Carbon