title: pH-dependent regulation of the multi-subunit cation/proton antiporter Pha1 system from Sinorhizobium meliloti.
abstract: The pha1 gene cluster (pha1A'-G) of Sinorhizobium meliloti has previously been characterized as a necessary component for proper invasion into plant root tissue. It has been suggested to encode a multi-subunit K(+)/H(+) antiporter, since mutations in the pha1 region rendered S. meliloti cells sensitive to K(+) and alkali, and because there is high amino acid sequence similarity to previously characterized multi-subunit cation/H(+) antiporters (Mrp antiporters). However, the detailed transport properties of the Pha1 system are yet to be determined. Interestingly, most of the Mrp antiporters are highly selective for Na(+), unlike the Pha1 system. Here, we report the functional expression of the Pha1 system in Escherichia coli and the measurement of cation/H(+) antiport activity. We showed that the Pha1 system is indeed a K(+)/H(+) antiporter with a pH optimum under mildly alkaline conditions. Moreover, we found that the Pha1 system can transport Na(+); this was unexpected based on previous phenotypic analyses of pha1 mutants. Furthermore, we demonstrated that the cation selectivity of the Pha1 system was altered when the pH was lowered from the optimum. The downregulation of Na(+)/H(+) and K(+)/H(+) antiport activities upon acidic shift appeared to occur via different processes, which might indicate the presence of distinct mechanisms for the regulation of the K(+)/H(+) and Na(+)/H(+) antiport activities of the Pha1 system.
authors.affiliation: Department of Microbiology, Faculty of Pharmacy, Niigata University of Pharmacy and Applied Life Sciences, Niigata-shi 956-8603, Japan.
authors.forename: Toshio
authors.initials: T
authors.lastname: Yamaguchi
authors.affiliation: Department of Microbiology, Faculty of Pharmacy, Niigata University of Pharmacy and Applied Life Sciences, Niigata-shi 956-8603, Japan.
authors.forename: Fuminori
authors.initials: F
authors.lastname: Tsutsumi
authors.affiliation: Department of Genetics and Molecular Biology, Faculty of Sciences, University of Pécs, H-7601 Pécs, Hungary.
authors.forename: Péter
authors.initials: P
authors.lastname: Putnoky
authors.affiliation: Department of Microbiology, Faculty of Pharmacy, Niigata University of Pharmacy and Applied Life Sciences, Niigata-shi 956-8603, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Fukuhara
authors.affiliation: Department of Microbiology, Faculty of Pharmacy, Niigata University of Pharmacy and Applied Life Sciences, Niigata-shi 956-8603, Japan.
authors.forename: Tatsunosuke
authors.initials: T
authors.lastname: Nakamura
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D005810:Multigene Family; D011188:Potassium / Q000378:metabolism; D027961:Potassium-Hydrogen Antiporters / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000235:genetics; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D008565:Membrane Proteins; C114817:PhaA protein, Sinorhizobium meliloti; D027961:Potassium-Hydrogen Antiporters; D011994:Recombinant Proteins; D012964:Sodium; D011188:Potassium