title: Salt stress enhances choline uptake in the halotolerant cyanobacterium Aphanothece halophytica.
abstract: The uptake of [14C]choline by a suspension of exponential-phase Aphanothece halophytica under various conditions has been studied. Salt stress was found to enhance the uptake of choline. The kinetics of choline transport followed the Michaelis-Menten relationship with apparent K(m) values of 272 and 286 microM, maximum rates of transport (V(max)) of 18 and 37 nmol/min/mg protein for unstressed and salt-stressed cells, respectively. Choline uptake under salt stress was significantly reduced in chloramphenicol-treated cells, suggesting that the activation by salt stress occurred via an inducible transport system. This was corroborated by the existence of the periplasmic choline binding protein, whose content was higher in cells grown under salt-stress condition. Exogenously provided choline significantly increased the growth rate of cells grown under salt stress, although less efficiently than glycine betaine. The presence of 1 mM choline in the growth medium conferred tolerance to high salinity on A. halophytica with the maintenance of high growth up to 1.5 M NaCl. The uptake of choline was Na(+)-dependent, sensitive to various metabolic inhibitors as well as thiol-reactive agents. The results of competition studies suggested that N-methyl on one end of molecule and on the other end either an aldehyde, an alcohol or a neutral group were important features for substrate recognition.
authors.affiliation: Laboratory of Biochemical Products, Department of Biochemistry, Faculty of Science, Chulalongkorn University, 10330, Bangkok, Thailand. iaran@sc.chula.ac.th
authors.forename: Aran
authors.initials: A
authors.lastname: Incharoensakdi
authors.forename: Aphichart
authors.initials: A
authors.lastname: Karnchanatat
mesh_terms: D001622:Betaine / Q000378:metabolism; D002701:Chloramphenicol; D002794:Choline / Q000378:metabolism* / Q000494:pharmacology; D000458:Cyanobacteria / Q000254:growth & development / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D008301:Maleimides; D026901:Membrane Transport Proteins / Q000032:analysis / Q000096:biosynthesis / Q000378:metabolism*; D009997:Osmotic Pressure; D019897:Periplasm / Q000378:metabolism; D012965:Sodium Chloride / Q000378:metabolism* / Q000494:pharmacology; D013997:Time Factors; D020245:p-Chloromercuribenzoic Acid
chemical_list: D008301:Maleimides; D026901:Membrane Transport Proteins; C065637:choline transporter; D001622:Betaine; D012965:Sodium Chloride; D020245:p-Chloromercuribenzoic Acid; D002701:Chloramphenicol; D002794:Choline