Nigericin-induced stimulation of photophosphorylation in chloroplasts

Amines have been shown recently to stimulate at low concentrations the steady-state rate of photophosphorylation by unbroken chloroplasts (Giersch, C. (1982) Z. Naturforsch. 37c, 242–250). In the present contribution it is demonstrated that not only amines but also the carboxylic ionophores nigericin and monensin at concentrations of 10 and 150 nM, respectively, stimulate the phosphorylation rate. The $\text{ATP}\text{2}\text{e}$ ratio is not decreased upon the addition of nigericin at concentrations that stimulate phosphorylation. Nigericin-induced stimulation is observed only in the presence of sufficient external potassium, indicating that the observed stimulation is unlikely to be a side-effect of the uncoupler but is related to H⁺-K⁺ exchange. The proton permeability of the thylakoid membrane is increased and the proton gradient decreased by amounts of nigericin that stimulate phosphorylation. The membrane potential is not affected in the steady state, indicating that the proton-motive force is slightly reduced upon addition of the ionophore. Data on the proton-motive force were related to maximum values of the phosphorylation potential, which was 45 000–50 000 M^?1 in the absence and 30 000–35 000 M^?1 in the presence of 10 nM nigericin. The observation that the $\text{ATP}\text{2}\text{e}$ ratio is not decreased in the presence of uncoupler-induced proton leakage is suggested to indicate that the thylakoid lumen does not represent a homogeneous phase of constant proton electrochemical potential. The results presented here are in agreement with the chemiosmotic concept as far as energetic aspects are concerned but seem to be at variance with the postulated free mobility of protons inside the thylakoids. A tentative model of uncoupler-induced stimulation of phosphorylation is presented.     

Cloning and endonuclease restriction analysis of argF and of the control region of the argECBH bipolar operon in Escherichia coli.

A 1.8 kb DNA fragment, liberated by endonuclease HindIII, contains the control region of the argECBH bipolar operon near one end and the weak secondary promoter of argH at the other extremity; it has been cloned in plasmid pBR322. The same plasmid vector has been used to clone the argF gene liberated from the chromosome by endonuclease BamHI. Restriction patterns for the two hybrid plasmids have been determined, using enzymes AluI, BglI, EcoRI, HaeIII, HincII, HindIII, HpaI and II, PstI and SalI. Two AluI sites situated on either side of and close to a HincII target delineate two short fragments covering the whole of the argECBH control region. The argF control elements are located in a region accessible to further dissection by BamHI, EcoRI, PstI and HindIII. Carriers of the argF plasmid produce extremely high amounts of ornithine carbamoyltransferase, a feature useful for purification of this enzyme.     

The insertion of the non-heme FeB cofactor into nitric oxide reductase from P. denitrificans depends on NorQ and NorD accessory proteins

Bacterial NO reductases (NOR) catalyze the reduction of NO into N₂O, either as a step in denitrification or as a detoxification mechanism. cNOR from Paracoccus (P.) denitrificans is expressed from the norCBQDEF operon, but only the NorB and NorC proteins are found in the purified NOR complex.Here, we established a new purification method for the P. denitrificans cNOR via a His-tag using heterologous expression in E. coli. The His-tagged enzyme is both structurally and functionally very similar to non-tagged cNOR. We were also able to express and purify cNOR from the structural genes norCB only, in absence of the accessory genes norQDEF. The produced protein is a stable NorCB complex containing all hemes and it can bind gaseous ligands (CO) to heme b₃, but it is catalytically inactive. We show that this deficient cNOR lacks the non-heme iron cofactor Fe_B. Mutational analysis of the nor gene cluster revealed that it is the norQ and norD genes that are essential to form functional cNOR. NorQ belongs to the family of MoxR P-loop AAA+ ATPases, which are in general considered to facilitate enzyme activation processes often involving metal insertion. Our data indicates that NorQ and NorD work together in order to facilitate non-heme Fe insertion. This is noteworthy since in many cases Fe cofactor binding occurs spontaneously. We further suggest a model for NorQ/D-facilitated metal insertion into cNOR.     

Exploring the in situ accessibility of small subunit ribosomal RNA of members of the domains Bacteria and Eukarya to oligonucleotide probes

The principle that the small subunit ribosomal RNA (ssu rRNA) is generally accessible to oligonucleotide probes designed to have high thermodynamic affinity was tested with Stenotrophomonas maltophilia, Rhodobacter sphaeroides, Bacillus subtilis, and Saccharomyces cerevisiae. Fluorescein-labeled probes, designed to have ΔG_overall° = −14 ± 1 and to avoid the potential of nucleobase-specific quenching, were used to target 20 randomly selected sites in each organism. A site was considered accessible if probe brightness was at least 10 times the background signal. With 30-h hybridizations, 71 out of 80 target sites passed the accessibility criterion. Three additional sites were demonstrated to be accessible with either longer hybridizations, which seemed to have a negative effect on some probes, or the addition of formamide to the hybridization buffer. The remaining 6 sites were demonstrated to be accessible by changing the fluorophore to Cy5, slightly modifying probe lengths, using dual-labeled fluorescein probes, or a combination of these approaches. Probe elongations were only needed in 4 probes, indicating a 95% success in correctly predicting ΔG_overall°, the key parameter for the design of high affinity probes. In addition, 94% of the fluorescein labeled probes yielded bright signals, demonstrating that nucleobase-specific quenching of fluorescein is an important factor affecting probe brightness that can be predicted during probe design. Overall, the results support the principle that with a rational design of probes, it is possible to make most target sites in the ssu rRNA accessible.     

N 6 -( 2 -isopentenyl) adenosine: biosynthesis in vitro in transfer RNA by an enzyme purified from Escherichia coli

An enzyme has been partially purified from Escherichia coli which catalyzes in vitro the transfer of the Δ²-isopentenyl group from Δ²-isopentenyl pyrophosphate to an adenosine residue in Mycoplasma sp. (Kid) tRNA. The product of the reaction is N⁶-(Δ²-isopentenyl) adenosine, which is known to be absent in this Mycoplasma tRNA. The enzyme has an approximate molecular weight of 55,000 daltons, requires reduced sulfhydryl groups and a divalent metal ion for full activity, and is specific for tRNA.     

Plasmodium berghei: inhibitors of ornithine decarboxylase block exoerythrocytic schizogony

DL-alpha-difluoromethylornithine and DL-alpha-monofluoromethyldehydroornithine methyl ester, inhibitors of ornithine decarboxylase, blocked exoerythrocytic schizogony of Plasmodium berghei in mice and in cultured human hepatoma cells. These effects were reversed by exogenous administration of the polyamine, spermidine. The antimalarial drug, primaquine, the side chain of which is structurally analogous to a natural polyamine, did not enhance the activity of alpha-difluoromethylornithine or alpha-monofluoromethyldehydroornithine methyl ester. These results extend previous observations that polyamines influence the malaria parasite's schizogony outside the red blood cell but not within it.     

High-performance liquid chromatographic analysis of porphyrins and their isomers with radial compression columns

Porphyrin methyl esters and the isomers of uroporphyrin and heptacarboxylic porphyrin were separated by high-performance liquid chromatography. Isocoproporphyrin was also separated from coproporphyrin. By slight modifications to the solvent mixture, the separation of all biological polycarboxylic porphyrins was achieved. These separations were made possible through the high efficiency of 10- or 5-μm particle-size Radial-PAK cartridges, which have been used in the separation of porphyrins in various excreta and tissues in a number of porphyrias.