After the transition, the crystal totally recovers its crystalline state and diffraction power. The symmetry is reduced from space-group I222 to its subgroup P21212 but the effects of this symmetry breaking on the structure are subtle.

DsbB is an E. coli membrane protein that oxidizes DsbA, the primary protein disulfide donor present in the periplasm. To understand how disulfide bonds are generated and introduced into secreted proteins, we determined the crystal structure of DsbB in a complex with DsbA and endogenous ubiquinone at 3.7 Å resolution. The first structure of DsbB revealed that DsbB contains the four-helix bundle scaffold in the transmembrane region and one short membrane-parallel α-helix in the long periplasmic loop. Strikingly, the disulfide-generating reaction center composed of Cys41, Cys44, Arg48 and ubiquinone is located near the N-terminus of the transmembrane helix 2, where oxidizing equivalents of ubiquinone are converted to a protein disulfide bond de novo. Whereas DsbB in the resting state contains a Cys104-Cys130 disulfide, Cys104 in the ternary complex is engaged in the intermolecular disulfide bond and captured by the hydrophobic groove of DsbA, resulting in its separation from Cys130. This DsbA-induced conformational change in DsbB seems to prevent the backward resolution of the complex and thereby promote the physiological electron flow from DsbA to DsbB. Recently, I examined functional roles of the membrane-parallel α-helix with strong amphiphilicity by systematic mutation analyses. Introduction of charged or helix-breaking residues into this region not only disrupted the peripheral membrane-association of this helix but also impaired DsbA oxidation activity of DsbB. On the basis of structural and biochemical data so far obtained, I propose the "cysteine relocation mechanism", by which DsbB oxidizes the extremely oxidizing (reduction-prone) dithiol oxidase, DsbA, efficiently.

The uptake of 10−1M–2.5 × 10−1M sulfate by roots and leaf slices of barley can be described by a single isotherm having, respectively, 8 and 5 phases with regularly increasing kinetic constants. Each phase covers a limited concentration range and obeys Michaelis‐Menten kinetics. The uptake of sulfate is proposed to be rate‐limited by a single mechanism or structure which is located in the plasmalemma (or cytoplasm) and which changes characteristics at certain discrete external concentrations (inflection points). Examination of published data indicates that the uptake of other inorganic ions by higher plant cells is also mediated by single, multi‐phasic mechanisms.

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Concentration-dependence and other characteristics of uptake of •'H-labeled L-lysine, L-methionine and L-proline by excised roots of barley {Hordeum vulgare L.) were studied. Use of relatively short uptake and wash periods and low solute concentrations ensured good estimates of influx across the plasmalemma.Uptake in the range of 10"' M -6,3 x 10"^ M can be precisely represented by four or five phases of single, multiphasic mechanisms. The mechanisms appear to be relatively specific as judged from the competition by unlabeled analogues. Structural requirements for interaction of a compound with the uptake site for methionine are given, as are the effects of analogues on the phase pattern for this amino acid. There is no indication of separate uptake and transition sites for methionine or lysine, i.e. phase transitions seem in this case to be caused by binding of molecule(s) to the uptake site.Uptake, but not phase patterns, was highly pH-dependent, The optima were pH 5 for lysine, pH 3-5 (a broad peak) for methionine and about pH 5.5 for proline. Uptake of the three amino acids was strongly inhibited by 2,4-dinitrophenol, sulfhydryl reagents and deoxycholate.

In the range 10−6M ‐ 5 × 10−2M uptake of K+ in excised roots of barley (Hordeum vulgare L. cv. Herta) with low and high K content could in both cases be represented by an isotherm with four phases. Uptake, especially in the range of the lower phases, was reduced in high K roots through decreases in Vmax and increases in Km. Similar data for other plants are also shown to be consistent with multiphasic kinetics. The concentrations at which transitions occurred were not affected by the K status, indicating the existence of separate uptake and transition sites. Uptake was markedly reduced in the presence of 10−5M 2,4‐dinitrophenol, especially at low K+ concentrations, but the isotherms remained multiphasic. This contraindicates major contributions from a non‐carrier‐mediated, passive flux. A tentative hypothesis for multiphasic ion uptake envisions a structure which changes conformation as a result of all‐or‐none changes in a separate transition site. The structure is “tight” at low external ion concentrations (low Vmax. low Km. active uptake, allosteric regulation) and “loose” at high concentrations (high Vmax‐ high Km‐ facilitated diffusion, no regulation).

Freshwater and terrestrial plants differ markedly in their ability to metabolize arsenate. In experiments with higher terrestrial plants, e.g. tomato, Lycopersicon esculentum Mill. cv. Better boy, 74As‐arsenate was readily taken up and reduced to arsenite. Methylation and reduction to methanearsonic acid, methanearsinic acid (indicated for the first time) and dimethylarsinic acid were apparent only in phosphate deficient plants. Lower and higher freshwater plants, e.g. Nitella tenuissima Kütz. and Lemna minima Phill., not only methylated arsenic but also produced considerable amounts of an arsoniumphospholipid previously identified in marine algae. These differences indicate that freshwater but not terrestrial plants have evolved mechanisms for rapid detoxication of arsenate, arsenite and other toxic arsenic species.

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