Immunological identification of yeast SCO1 protein as a component of the inner mitochondrial membrane

Summary The SCO1 gene of Saccharomyces cerevisiae encodes a 30 kDa protein which is specifically required for a post-translational step in the accumulation of subunits 1 and 2 of cytochrome c oxidase (COXI and COXII). Antibodies directed against a -Gal::SCO1 fusion protein detect SCO1 in the mitochondrial fraction of yeast cells. The SCO1 protein is an integral membrane protein as shown by its resistance to alkaline extraction and by its solubilization properties upon treatment with detergents. Based on the results obtained by isopycnic sucrose gradient centrifugation and by digitonin treatment of mitochondria, SCO1 is a component of the inner mitochondrial membrane. Membrane localization is mediated by a stretch of 17 hydrophobic amino acids in the amino-terminal region of the protein. A truncated SCO1 derivative lacking this segment, is no longer bound to the membrane and simultaneously loses its biological function. The observation that membrane localization of SCO1 is affected in mitochondria of a rho ⁰ strain, hints at the possible involvement of mitochondrially coded components in ensuring proper membrane insertion.     

Growth and photosynthetic response of nine tropical species with long-term exposure to elevated carbon dioxide

Summary Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO₂ for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO₂ concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C₃ metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO₂ was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C₃ species tested and in one CAM species, Aechmea magdalenae at high CO₂. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C₄ grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C₃ species suggest that elevated CO₂ may increase photosynthetic rate relative to ambient CO₂ over a wide range of light conditions. The response of CO₂ assimilation to internal C_i suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO₂. This experiment suggests that: (1) a global rise in CO₂ may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO₂.     

Proton and sucrose transport in isolated tonoplast vesicles from sugarcane stalk tissue

Tonoplast vesicles prepared from immature sugarcane ( Saccharum spp., hybrid cv. H65–7052) tissue and purified on a discontinuous dextran gradient take up sucrose. Uptake was stimulated by MgATP. Evidence that the mechanism is linked to proton transport is derived from "pH jump'data and from inhibition of ATP-stimulated sucrose transport by the protonophore carbonyl cyanide m -chlorophenylhydrazone (CCCP) and by the proton-channel blocker of proton-linked ATPases. N. N '-dicyclo-hexylcarbodiimide (DCCD). A saturable phase of sucrose uptake was found at low substrate concentrations, and a linear phase characterized uptake at higher concentrations. Uptake was specific for sucrose, as demonstrated by competition experiments with various sugars. Sucrose uptake by the vesicle fraction was inhibited by KNO₃, protonophores and protein modifying reagents, whereas sodium orthovanadate had no effect. Overall, the evidence suggests an ATP-hydrolysis-dependent tonoplasl antiport for sucrose transport, although a more direct influence of ATP on conformational changes in relevant tonoplast proteins cannot be ruled out.     

Current perspectives on plasmodesmata: structure and function

Recent studies on plasmodesmata have shown that these important intercellular passages for communication and transport are much more sophisticated in both structure and regulatory abilities than previously imagined. A complex, but not well understood, substructure has been revealed by a variety of increasingly reliable ultrastructural techniques. Proteinaceous particles are seen within the cytoplasmic sleeve surrounding the desmotubule. Dye-coupling studies have provided experimental evidence for the physical pathway of solute movement, supporting conclusions about substructural dimensions within plasmodesmata drawn from the ultrastructural studies. Calcium has been identified as a major factor in the regulation of intercellular communication via plasmodesmata. Evidence from studies on virus movement through plasmodesmata suggests a direct interaction between virallycoded movement proteins and plasmodesmata in the systemic spread of many viruses. There is increasing evidence, albeit indirect, that in some plant species phloem loading may involve transport of photoassimilate entirely within the symplast from mesophyll cells to the sieve element-companion cell complexes of minor veins.     

Cardenolides from Asclepias asperula subsp. Capricornu and A. Viridis

6′-O-(E-4-hydroxycinnamoyl) Desglucouzarin, the first cardenolide containing a cinnamoyl ester moiety, has been isolated from the ethanolic extract of the milkweed, Asclepias asperula. In addition, five known cardenolides were isolated and identified from A. asperula and A. viridis.     

Clerodane diterpenoids from the roots of Portulaca pilosa

Three trans-clerodane diterpenoids, pilosanol A, B and C, the last compound being a glucoside, have been isolated from the roots of Portulaca pilosa. They show a marked contrast in skeletal type with the constituents of aerial part. Evolutionary changes in the biosynthetic abilities of Portulaca species is discussed.