title: Expression of genes involved in the uptake of inorganic carbon in the gill of a deep-sea vesicomyid clam harboring intracellular thioautotrophic bacteria.
abstract: Deep-sea vesicomyid clams, including the genus Phreagena (formerly Calyptogena), harbor thioautotrophic bacterial symbionts in the host symbiosome, which consists of cytoplasmic vacuoles in gill epithelial cells called bacteriocytes. The symbiont requires inorganic carbon (Ci), such as CO2, HCO3(-), and CO3(2-), to synthesize organic compounds, which are utilized by the host clam. The dominant Ci in seawater is HCO3(-), which is impermeable to cell membranes. Within the bacteriocyte, cytoplasmic carbonic anhydrase (CA) from the host, which catalyzes the inter-conversion between CO2 and HCO3(-), has been shown to be abundant and is thought to supply intracellular CO2 to symbionts in the symbiosome. However, the mechanism of Ci uptake by the host gill from seawater is poorly understood. To elucidate the influx pathway of Ci into the bacteriocyte, we isolated the genes related to Ci uptake via the pyrosequencing of cDNA from the gill of Phreagena okutanii, and investigated their expression patterns. Using phylogenetic and amino acid sequence analyses, three solute carrier family 4 (SLC4) bicarbonate transporters (slc4co1, slc4co2, and slc4co4) and two membrane-associated CAs (mcaco1 and mcaco2) were identified as candidate genes for Ci uptake. In an in situ hybridization analysis of gill sections, the expression of mcaco1 and mcaco2 was detected in the bacteriocytes and asymbiotic non-ciliated cells, respectively, and the expression of slc4co1 and slc4co2 was detected in the asymbiotic cells, including the intermediate cells of the inner area and the non-ciliated cells of the external area. Although subcellular localizations of the products of these genes have not been fully elucidated, they may play an important role in the uptake of Ci into the bacteriocytes. These findings will improve our understanding of the Ci transport system in the symbiotic relationships of chemosynthetic bivalves.
authors.affiliation: Department of Research Center of Aquatic Genomics, National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan; Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan; Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo, 108-8477, Japan. Electronic address: hongoy@affrc.go.jp.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Hongo
authors.affiliation: Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. Electronic address: teikuta@jamstec.go.jp.
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Ikuta
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. Electronic address: takakiy@jamstec.go.jp.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Takaki
authors.affiliation: Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. Electronic address: shimas@jamstec.go.jp.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Shimamura
authors.affiliation: Functional Genomics Facility, National Institute for Basic Biology, Okazaki 444-8585, Japan. Electronic address: shige@nibb.ac.jp.
authors.forename: Shuji
authors.initials: S
authors.lastname: Shigenobu
authors.affiliation: Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan; Research and Development Center for Marine Biosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. Electronic address: tadashim@jamstec.go.jp.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Maruyama
authors.affiliation: Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan; Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo, 108-8477, Japan. Electronic address: tyoshida@jamstec.go.jp.
authors.forename: Takao
authors.initials: T
authors.lastname: Yoshida
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D049872:Bivalvia / Q000235:genetics* / Q000378:metabolism; D002244:Carbon / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D005880:Gills / Q000378:metabolism*; D017403:In Situ Hybridization; D008386:Marine Biology; D008969:Molecular Sequence Data; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid
chemical_list: D001639:Bicarbonates; D002352:Carrier Proteins; D002244:Carbon
keywords: Bacteriocyte; Bivalve; Inorganic carbon acquisition; Membrane-associated carbonic anhydrase; Solute carrier family 4; Symbiosis