Salt impact on photosynthesis and leaf ultrastructure of <Emphasis Type="Italic">Aeluropus littoralis</Emphasis>

The effects of salinity (400 mM NaCl) on growth, biomass partitioning, photosynthesis, and leaf ultrastructure were studied in hydroponically grown plants of Aeluropus littoralis (Willd) Parl. NaCl produced a significant inhibition of the main growth parameters and a reduction in leaf gas exchange (e.g. decreased rates of photosynthesis and stomatal conductance). However, NaCl salinity affected neither the composition of photosynthesis pigments nor leaf water content. The reduction in leaf gas exchange seemed to correlate with a decrease in mesophyll thickness as well as a severe disorganisation of chloroplast structure, with misshapen chloroplasts and dilated thylakoid membranes. Conspicuously, mesophyll chloroplasts were more sensitive to salt treatment than those of bundle sheath cells. The effects of NaCl toxicity on leaf structure and ultrastructure and the associated physiological implications are discussed in relation to the degree of salt resistance of A. littoralis.     

Molecular Cloning and Functional Analysis of a Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> Antiporter Gene <Emphasis Type="Italic">ThNHX1</Emphasis> from a Halophytic Plant <Emphasis Type="Italic">Thellungiella halophila</Emphasis>

Na⁺/H⁺ exchanger catalyzes the countertransport of Na⁺ and H⁺ across membranes. Using the rapid amplification of cDNA ends method, a Na⁺/H⁺ antiporter gene (ThNHX1) was isolated from a halophytic plant, salt cress (Thellungiella halophila). The deduced amino acid sequence contained 545 amino acid residues with a conserved amiloride-binding domain (⁸⁷LFFIYLLPPI⁹⁶) and shared more than 94% identity with that of AtNHX1 from Arabidopsis thaliana. The ThNHX1 mRNA level was upregulated by salt and other stresses (abscisic acid, polyethylene glycol, and high temperature). This gene partially complemented the Na⁺/Li⁺-sensitive phenotype of a yeast mutant that was deficient in the endosomal–vacuolar Na⁺/H⁺ antiporter ScNHX1. Overexpression of ThNHX1 in Arabidopsis increased salt tolerance of transgenic plants compared with the wild-type plants. In addition, the silencing of ThNHX1 gene in T. halophila caused the transgenic plants to be more salt and osmotic sensitive than wild-type plant. Together, these results suggest that ThNHX1 may function as a tonoplast Na⁺/H⁺ antiporter and play an important role in salt tolerance of T. halophila. Chunxia Wu, Xiuhua Gao, and Xiangqiang Kong contributed equally to this work.     

Pivotal Role of Reduced Glutathione in Oxygen-induced Regulation of the Na<Superscript><Emphasis Type="Bold">+</Emphasis></Superscript>/K<Superscript><Emphasis Type="Bold">+</Emphasis></Superscript> Pump in Mouse Erythrocyte Membranes

This study addresses the mechanisms of oxygen-induced regulation of ion transport pathways in mouse erythrocyte, specifically focusing on the role of cellular redox state and ATP levels. Mouse erythrocytes possess Na⁺/K⁺ pump, K⁺-Cl^– and Na⁺-K⁺-2Cl^– cotransporters that have been shown to be potential targets of oxygen. The activity of neither cotransporter changed in response to hypoxia-reoxygenation. In contrast, the Na⁺/K⁺ pump responded to hypoxic treatment with reversible inhibition. Hypoxia-induced inhibition was abolished in Na⁺-loaded cells, revealing no effect of O₂ on the maximal operation rate of the pump. Notably, the inhibitory effect of hypoxia was not followed by changes in cellular ATP levels. Hypoxic exposure did, however, lead to a rapid increase in cellular glutathione (GSH) levels. Decreasing GSH to normoxic levels under hypoxic conditions abolished hypoxia-induced inhibition of the pump. Furthermore, GSH added to the incubation medium was able to mimic hypoxia-induced inhibition. Taken together these data suggest a pivotal role of intracellular GSH in oxygen-induced modulation of the Na⁺/K⁺ pump activity.     

Inward-rectifier K<Superscript><Emphasis Type="Bold">+</Emphasis></Superscript> Current in Guinea-pig Ventricular Myocytes Exposed to Hyperosmotic Solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K⁺ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K⁺ current (I_K1), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrodes solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3–2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba²⁺-sensitive I_K1, an increase in the amplitude of outward I_K1, and a steepening of the slope of the inward I_K1-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G_K1) and rectification, G_K1-V data were fitted with Boltzmann functions to determine maximal G_K1 (G_K1max) and voltage at one-half G_K1max (V_0.5). Superfusion with hyperosmotic sucrose solutions led to significant increases in G_K1max (e.g., 28±2% with 1.8T), and significant negative shifts in V_0.5 (e.g., –6.7±0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G_K1max and V_0.5 were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K⁺. We conclude that the effects of hyperosmotic sucrose solutions on I_K1 are related to shrinkage-induced concentrating of intracellular K⁺.     

K<Superscript>+</Superscript> Channels in Cardiomyocytes of the Pulmonate Snail <Emphasis Type="Italic">Helix</Emphasis>

We used the patch-clamp technique to identify and characterize the electrophysiological, biophysical, and pharmacological properties of K⁺ channels in enzymatically dissociated ventricular cells of the land pulmonate snail Helix. The family of outward K⁺ currents started to activate at –30 mV and the activation was faster at more depolarized potentials (time constants: at 0 mV 17.4 ± 1.2 ms vs. 2.5 ± 0.1 ms at + 60 mV). The current waveforms were similar to those of the A-type family of voltage-dependent K⁺ currents encoded by Kv4.2 in mammals. Inactivation of the current was relatively fast, i.e., 50.2 ± 1.8% of current was inactivated within 250 ms at + 40 mV. The recovery of K⁺ channels from inactivation was relatively slow with a mean time constant of 1.7 ± 0.2 s. Closer examination of steady-state inactivation kinetics revealed that the voltage dependency of inactivation was U-shaped, exhibiting less inactivation at more depolarized membrane potentials. On the basis of this phenomenon, we suggest that a channel encoded by Kv2.1 similar to that in mammals does exist in land pulmonates of the Helix genus. Outward currents were sensitive to 4-aminopyridine and tetraethylammonium chloride. The last compound was most effective, with an IC₅₀ of 336 ± 142 µmol l^–1. Thus, using distinct pharmacological and biophysical tools we identified different types of voltage-gated K⁺ channels. Present address for S.A.K.: Brigham and Womens Hospital, Cardiovascular Division, Harvard Medical School, 75 Francis St., Thorn 1216, Boston, MA 02115.     

NhaK,a novel monovalent cation/H<Superscript>+</Superscript> antiporter of <Emphasis Type="Italic">Bacillus subtilis</Emphasis>

Four Na⁺/H⁺ antiporters, Mrp, TetA(L), NhaC, and MleN have so far been described in Bacillus subtilis 168. We identified an additional Na⁺/H⁺ antiporter, YvgP, from B. subtilis that exhibits homology to the cation: proton antiporter-1 (CPA-1) family. The yvgP-dependent complementation observed in a Na⁺(Ca²⁺)/H⁺ antiporter-defective Escherichia coli mutant (KNabc) suggested that YvgP effluxed Na⁺ and Li⁺. In addition, effects of yvgP expression on a K⁺ uptake-defective mutant of E. coli indicated that YvgP also supported K⁺ efflux. In a fluorescence-based assay of everted membrane vesicles prepared from E. coli KNabc transformants, YvgP-dependent Na⁺ (K⁺, Li⁺, Rb⁺)/H⁺ antiport activity was demonstrated. Na⁺ (K⁺, Li⁺)/H⁺ activity was higher at pH 8.5 than at pH 7.5. Mg²⁺, Ca²⁺ and Mn²⁺ did not serve as substrates but they inhibited YvgP antiport activities. Studies of yvgP expression in B. subtilis, using a reporter gene fusion, showed a significant constitutive level of expression that was highest in stationary phase, increasing as stationary phase progressed. In addition, the expression level was significantly increased in the presence of added K⁺ and Na⁺.     

Transport of Na<Superscript>+</Superscript> and K<Superscript>+</Superscript> by an antiporter-related subunit from the <Emphasis Type="Italic">Escherichia coli </Emphasis>NADH dehydrogenase I produced in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

The NADH dehydrogenase I from Escherichia coli is a bacterial homolog of the mitochondrial complex I which translocates Na⁺ rather than H⁺. To elucidate the mechanism of Na⁺ transport, the C-terminally truncated NuoL subunit (NuoL_N) which is related to Na⁺/H⁺ antiporters was expressed as a protein A fusion protein (ProtA–NuoL_N) in the yeast Saccharomyces cerevisiae which lacks an endogenous complex I. The fusion protein inserted into membranes from the endoplasmatic reticulum (ER), as confirmed by differential centrifugation and Western analysis. Membrane vesicles containing ProtA–NuoL_N catalyzed the uptake of Na⁺ and K⁺ at rates which were significantly higher than uptake by the control vesicles under identical conditions, demonstrating that ProtA–NuoL_N translocated Na⁺ and K⁺ independently from other complex I subunits. Na⁺ transport by ProtA–NuoL_N was inhibited by EIPA (5-(N-ethyl-N-isopropyl)-amiloride) which specifically reacts with Na⁺/H⁺ antiporters. The cation selectivity and function of the NuoL subunit as a transporter module of the NADH dehydrogenase complex is discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Freshwater to seawater acclimation of juvenile bull sharks (<Emphasis Type="Italic">Carcharhinus leucas</Emphasis>): plasma osmolytes and Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase activity in gill,rectal gland,kidney and intestine

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l^–1 kg^–1) were acclimated to SW (980–1,000 mOsm l^–1 kg^–1) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l^–1 kg^–1. In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l^–1 kg^–1) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na⁺, Cl^–, K⁺, Mg²⁺, Ca²⁺, urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na⁺/K⁺-ATPase activity. Na⁺/K⁺-ATPase activity in the gills and intestine was less than 1 mmol Pi mg^–1 protein h^–1 and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na⁺/K⁺-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na⁺/K⁺-ATPase activity was 5.6±0.8 and 9.2±0.6 mmol Pi mg^–1 protein h^–1, respectively. Na⁺/K⁺-ATPase activity in the kidney of FW and SW acclimated animals was 8.4±1.1 and 3.3±1.1 Pi mg^–1 protein h^–1, respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.     

Cloning and characterization of a Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> antiporter gene of the moderately halophilic bacterium <Emphasis Type="Italic">Halobacillus aidingensis</Emphasis> AD-6<Superscript>T</Superscript>

A gene encoding a Na(+)/H(+) antiporter was obtained from the genome of Halobacillus aidingensis AD-6(T), which was sequenced and designated as nhaH. The deduced amino acid sequence of the gene was 91% identical to the NhaH of H. dabanensis, and shared 54% identity with the NhaG of Bacillus subtilis. The cloned gene enable the Escherichia coli KNabc cell, which lack all of the major Na(+)/H(+) antiporters, to grow in medium containing 0.2 M NaCl or 10 mM LiCl. The nhaH gene was predicted to encode a 43.5 kDa protein (403 amino acid residues) with 11 putative transmembrane regions. Everted membrane vesicles prepared from E. coli KNabc cells carrying NhaH exhibited Na(+)/H(+) as well as Li(+)/H(+) antiporter activity, which was pH-dependent with the highest activity at pH 8.0, and no K(+)/H(+) antiporter activity was detected. The deletion of hydrophilic C-terminal amino acid residues showed that the short C-terminal tail was vital for Na(+)/H(+) antiporter activity.     

The Na<Superscript>+</Superscript>-translocating ATPase in the plasma membrane of the marine microalga<Emphasis Type="Italic"> Tetraselmis viridis</Emphasis> catalyzes Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> exchange

Our previous investigations have established that Na⁺ translocation across the Tetraselmis viridis plasma membrane (PM) mediated by the primary ATP-driven Na⁺-pump, Na⁺-ATPase, is accompanied by H⁺ counter-transport [Y.V. Balnokin et al. (1999) FEBS Lett 462:402–406]. The hypothesis that the Na⁺-ATPase of T. viridis operates as an Na⁺/H⁺ exchanger is tested in the present work. The study of Na⁺ and H⁺ transport in PM vesicles isolated from T. viridis demonstrated that the membrane-permeant anion NO₃^– caused (i) an increase in ATP-driven Na⁺ uptake by the vesicles, (ii) an increase in (Na⁺+ATP)-dependent vesicle lumen alkalization resulting from H⁺ efflux out of the vesicles and (iii) dissipation of electrical potential, , generated across the vesicle membrane by the Na⁺-ATPase. The (Na⁺+ATP)-dependent lumen alkalization was not significantly affected by valinomycin, addition of which in the presence of K⁺ abolished at the vesicle membrane. The fact that the Na⁺-ATPase-mediated alkalization of the vesicle lumen is sustained in the absence of the transmembrane is consistent with a primary role of the Na⁺-ATPase in driving H⁺ outside the vesicles. The findings allowed us to conclude that the Na⁺-ATPase of T. viridis directly performs an exchange of Na⁺ for H⁺. Since the Na⁺-ATPase generates electric potential across the vesicle membrane, the transport stoichiometry is mNa⁺/nH⁺, where m>n.Abbreviations BTP Bis-Tris-Propane, 1,3-bis[tris(hydroxymethyl)methylamino]-propane - CCCP Carbonyl cyanide m-chlorophenylhydrazone - DTT Dithiothreitol - NCDC 2-Nitro-4-carboxyphenyl N,N-diphenylcarbamate - PMSF Phenylmethylsulfonyl fluoride - PM Plasma membrane     

Cloning of two SOS1 transporters from the seagrass <Emphasis Type="Italic">Cymodocea nodosa</Emphasis>. SOS1 transporters from <Emphasis Type="Italic">Cymodocea</Emphasis> and <Emphasis Type="Italic">Arabidopsis</Emphasis> mediate potassium uptake in bacteria

Two cDNAs isolated from Cymodocea nodosa, CnSOS1A, and CnSOS1B encode proteins with high-sequence similarities to SOS1 plant transporters. CnSOS1A expressed in a yeast Na⁺-efflux mutant under the control of a constitutive expression promoter mimicked AtSOS1 from Arabidopsis; the wild type cDNA did not improve the growth of the recipient strain in the presence of Na⁺, but a cDNA mutant that expresses a truncated protein suppressed the defect of the yeast mutant. In similar experiments, CnSOS1B was not effective. Conditional expression, under the control of an arabinose responsive promoter, of the CnSOS1A and CnSOS1B cDNAs in an Escherichia coli mutant defective in Na⁺ efflux was toxic, and functional analyses were inconclusive. The same constructs transformed into an E. coli K⁺-uptake mutant revealed that CnSOS1A was also toxic, but that it slightly suppressed defective growth at low K⁺. Truncation in the C-terminal hydrophilic tail of CnSOS1A relieved the toxicity and proved that CnSOS1A was an excellent low-affinity K⁺ and Rb⁺ transporter. CnSOS1B mediated a transient, extremely rapid K⁺ or Rb⁺ influx. Similar tests with AtSOS1 revealed that it was not toxic and that the whole protein exhibited excellent K⁺ and Rb⁺ uptake characteristics in bacteria.     

Phosphorylation Regulates an Inwardly Rectifying ATP-sensitive K<Superscript><Emphasis Type="Bold">+</Emphasis></Superscript>- Conductance in Proximal Tubule Cells of Frog Kidney

K⁺ channels in the renal proximal tubule play an important role in salt reabsorption. Cells of the frog proximal tubule demonstrate an inwardly rectifying, ATP-sensitive K⁺ conductance that is inhibited by Ba²⁺, G_Ba. In this paper we have investigated the importance of phosphorylation state on the activity of G_Ba in whole-cell patches. In the absence of ATP, G_Ba decreased over time; this fall in G_Ba involved phosphorylation, as rundown was inhibited by alkaline phosphatase and was accelerated by the phosphatase inhibitor F⁻(10 mM). Activation of PKC using the phorbol ester PMA accelerated rundown via a mechanism that was dependent on phosphorylation. In contrast, the inactive phorbol ester PDC slowed rundown. Inclusion of the PKC inhibitor PKC-ps in the pipette inhibited rundown. These data indicate that PKC-mediated phosphorylation promotes channel rundown. Rundown was prevented by the inclusion of PIP-2 in the pipette. PIP-2 also abrogated the PMA-mediated increase in rundown, suggesting that regulation of G_Ba by PIP-2 occurred downstream of PKC-mediated phosphorylation. G-protein activation inhibited G_Ba, with initial currents markedly reduced in the presence of GTPγs. These properties are consistent with G_Ba being a member of the ATP-sensitive K⁺ channel family.     

Structure and Mechanism of Vacuolar Na<Superscript>+</Superscript>-Translocating ATPase From <Emphasis Type="Italic">Enterococcus hirae</Emphasis>

V-type Na⁺-ATPase from Entercoccus hirae consists of nine kinds of subunits (NtpA₃, B₃, C₁, D₁, E₁₋₃, F₁₋₃, G₁, I₁, and K₁₀) which are encoded by the ntp operon. The amino acid sequences of the major subunits, A, B, and K (proteolipid), were highly similar to those of A, B, and c subunits of eukaryotic V-ATPases, and those of β, α, and c subunits of F-ATPases. We modeled the A and B subunits by homology modeling using the structure of β and α subunits of F-ATPase, and obtained an atomic structure of NtpK ring by X-ray crystallography. Here we briefly summarize our current models of the whole structure and mechanism of the E. hirae V-ATPase.     

Regulation of expression of Na<Superscript>+</Superscript>-translocating NADH:quinone oxidoreductase genes in <Emphasis Type="Italic">Vibrio harveyi</Emphasis> and <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis>

The expression of genes encoding sodium-translocating NADH:quinone oxidoreductase (Na⁺-NQR) was studied in the marine bacterium Vibrio harveyi and in the enterobacterium Klebsiella pneumoniae. It has been shown that such parameters as NaCl concentration, pH value, and presence of an uncoupler in the growth media do not influence significantly the level of nqr expression. However, nqr expression depends on the growth substrates used by these bacteria. Na⁺-NQR is highly repressed in V. harveyi during anaerobic growth, and nqr expression is modulated by electron acceptors and values of their redox potentials. The latter effect was shown to be independent of the ArcAB regulatory system. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Accession number: EF394942 (Vibrio harveyi arcB gene, partial cds).     

Enhancing yield of <Emphasis Type="Italic">S</Emphasis>-adenosylmethionine in <Emphasis Type="Italic">Pichia pastoris</Emphasis> by controlling NH<Subscript>4</Subscript><Superscript>+</Superscript> concentration

Yield of S-adenosylmethionine was improved significantly in recombinant Pichia pastoris by controlling NH₄ ⁺ concentration. The highest production rate was 0.248 g/L h when NH₄ ⁺ concentration was 450 mmol/L and no repression of cell growth was observed. Within very short induction time (47 h), 11.63 g/L SAM was obtained in a 3.7 L bioreactor.     

Involvement of Long-Distance Na<Superscript>+</Superscript> Transport in Maintaining Water Potential Gradient in the Medium-Root-Leaf System of a Halophyte <Emphasis Type="Italic">Suaeda altissima</Emphasis>

The aim of this study was to determine the range of NaCl concentrations in the nutrient solution that allow Suaeda altissima (L.) Pall., a salt-accumulating halophyte, to maintain the upward gradient of water potential in the “medium-root-leaf” system. We evaluated the contribution of Na⁺ ions in the formation of water potential gradient and demonstrated that Na⁺ loading into the xylem is involved in this process. Plants were grown in water culture at NaCl concentrations ranging from zero to 1 M. The water potential of leaf and root cells was measured with the method of isopiestic thermocouple psychrometry. When NaCl concentration in the growth medium was raised in the range of 0–500 mM (the medium water potential was lowered accordingly), the root and leaf cells of S. altissima decreased their water potential, thus promoting the maintenance of the upward water potential gradient in the medium-root-leaf system. Growing S. altissima at NaCl concentrations f 750 mM and 1 M disordered water homeostasis and abolished the upward gradient of water potential between roots and leaves. At NaCl concentrations of 0–250 mM, the detached roots of S. altissima were capable of producing the xylem exudate. The concentration of Na⁺ in the exudate was 1.3 to 1.6 times higher than in the nutrient medium; the exudate pH was acidic and was lowered from 5.5 to 4.5 with the rise in the salt concentration. The results indicate that the long-distance Na⁺ transport and, especially, the mechanism of Na⁺ loading into the xylem play a substantial role in the formation of water potential gradient in S. altissima. The accumulation of Na⁺ in the xylem and acidic pH values of the xylem sap suggest that Na⁺ loading into the xylem is carried out by the Na⁺/H⁺ antiporter of the plasma membrane in parenchymal cells of the root stele.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 549–557.Original Russian Text Copyright © 2005 by Balnokin, Kotov, Myasoedov, Khailova, Kurkova, Lun’kov, Kotova.     

Purification and characterization of ferredoxin-NADP<Superscript>+</Superscript> reductase encoded by <Emphasis Type="Italic">Bacillus subtilis yumC</Emphasis>

From Bacillus subtilis cell extracts, ferredoxin-NADP⁺ reductase (FNR) was purified to homogeneity and found to be the yumC gene product by N-terminal amino acid sequencing. YumC is a 94-kDa homodimeric protein with one molecule of non-covalently bound FAD per subunit. In a diaphorase assay with 2,6-dichlorophenol-indophenol as electron acceptor, the affinity for NADPH was much higher than that for NADH, with K_m values of 0.57 M vs >200 M. K_cat values of YumC with NADPH were 22.7 s^–1 and 35.4 s^–1 in diaphorase and in a ferredoxin-dependent NADPH-cytochrome c reduction assay, respectively. The cell extracts contained another diaphorase-active enzyme, the yfkO gene product, but its affinity for ferredoxin was very low. The deduced YumC amino acid sequence has high identity to that of the recently identified Chlorobium tepidum FNR. A genomic database search indicated that there are more than 20 genes encoding proteins that share a high level of amino acid sequence identity with YumC and which have been annotated variously as NADH oxidase, thioredoxin reductase, thioredoxin reductase-like protein, etc. These genes are found notably in gram-positive bacteria, except Clostridia, and less frequently in archaea and proteobacteria. We propose that YumC and C. tepidum FNR constitute a new group of FNR that should be added to the already established plant-type, bacteria-type, and mitochondria-type FNR groups.