PMC

Generation time and PMF parameters of B. pseudofirmus OF4 as a function of external pH in continuous cultures of malate-containing medium. Cells were grown in semi-defined malate containing medium maintained at the indicated pH values. tg, doubling time was calculated from the dilution rates in the chemostat and PMF parameters were determined as described in reference []; the data are replotted from Figure 1 of this reference with permission from Blackwell Publishing Ltd.

Overall architecture of NhaA. A ribbon presentation of the NhaA molecule viewed parallel to the membrane (grey broken line) is shown. The 12 TMSs are labeled with Roman numerals. N and C indicate the N and C termini. Note the unique and novel fold formed by unwinding of helices VI and XI in the middle of the membrane. The remaining short helices at the periplasmic or cytoplasmic sides of the membrane are denoted by p or c respectively. The cytoplasmic and periplasmic funnels are depicted by black line. For further details see [].

Alkaline pH shift experiments with E. coli and alkaliphilic B. pseudofirmus RAB. A. E. coli cells were grown logarithmically at pH 7.2 in minimal glycerol medium batch cultures before the pH of the medium was adjusted to 8.3 over 30 seconds. A rapid method of cytoplasmic pH and ΔΨ determination in intact cells has made it possible to follow the time course and sequence of changes occuring following the pH shift []. Growth was also assed (Klett units). The data are replotted from Figure 3 in reference [] with permission from the American Society for Microbiology. B. Washed pH 10.5-grown cells of B. pseudofirmus RAB were equilibrated in potassium carbonate buffer at pH 8.5 and then rapidly shifted to pH 10.5 buffer with either 50 mM potassium carbonate (“no Na⁺”, with or without added AIB), 50 mM sodium carbonate (“Na⁺”) or 50 mM sodium carbonate + 500 μM AIB. The cytoplasmic pH was monitored after the shift. The figure is a modified version of from reference [], with permission from the American Society for Biochemistry and Molecular Biology.

Na⁺/H⁺ antiporters, the Na⁺ cycle and selected H⁺- and K⁺-translocating proteins in membranes of four physiologically distinct bacteria. Transporters of the inner membrane of two Gram-negative neutralophilic examples, E. coli and V. cholerae, are schematically depicted within the inner membrane surrounded by the outer membrane. E. coli: a respiratory chain with Q representing ubiquinone or meanquinone, ATP synthase (F₀F₁) and flagellar motor that are all H⁺-coupled [, ]; both H⁺- and Na⁺-coupled solute symporters [, ]; and four monovalent cation/proton antiporters, one of which is also a multidrug/proton antiporter [, , , ]. V. cholerae: a respiratory chain that extrudes both Na⁺ and H⁺ and H⁺-coupled ATP synthase; Na⁺-coupled solute symporters and polar flagellar motor [, ] (whereas marine Vibrio species also have H⁺-coupled lateral flagella []); and at least five monovalent cation/proton antiporters; an annotated NhaP and MleN are not shown ([, , ], www.membranetransport.org). Transporters in Gram-positive neutralophilic B. subtilis and the alkaliphilic species B. pseudofirmus OF4 and Bacillus halodurans C-125 are shown in the cytoplasmic membrane surrounded by a peptidoglycan layer that has associated SCWP (secondary cell wall polymers). The Bacillus species have H⁺-extruding respiratory chains (MQ=menaquinone), H⁺-coupled ATP synthases [, , ], an ABC-type Na⁺ efflux system [, ], and a CPA:2-type K⁺-extruding system that has been shown to also extrude NH₄⁺ in B. pseudofirmus OF4 [, ]. B. subtilis: additionally has both H⁺- and Na⁺-coupled solute symporters and flagellar motors [, ], five characterized monovalent cation/proton antiporters [–, , ] and two Ktr-type K⁺- uptake systems whose ion-coupling properties are not established []. B. pseudofirmus OF4 has exclusively Na⁺-coupled solute symporters and flagellar rotors [, ], two monovalent cation/proton antiporters thus far partially characterized [] and a voltage-gated Na⁺ channel that plays a role in supporting pH homeostasis as well as chemotaxis and motility[]. The B. halodurans C-125 genome indicates the presence of MleN and a CPA1 type antiporter [].