Role of peroxidase and esterase activities during cotton fiber development

Cytoplasmic and salt-extracted wall peroxidase and nonspecific esterase activities along with growth analysis were investigated during the entire period of cotton fiber development. Both the peroxidase fractions, when assayed with chlorogenic and ferulic acids as substrates, recorded low levels during the fiber elongation phase, and a close relationship between cessation of elongation growth and increase in peroxidase activity was discernible. Nonspecific esterase activity in both cytoplasmic and salt-extracted fractions, on the other hand, showed higher activity during the elongation phase, whereas during the secondary thickening phase it decreased. The role of cytoplasmic peroxidase in IAA oxidation is discussed. It is suggested that esterases and peroxidases associated with wall fractions may well be involved in turnover of phenolic acids that are cross-linked to wall polysaccharides.     

Relative Impacts of Elephant and Fire on Large Trees in a Savanna Ecosystem

Elephant and fire are considered to be among the most important agents that can modify the African savanna ecosystem. Although the synergistic relationship between these two key ecological drivers is well documented, it has proved much more difficult to establish the relative effects they have on savanna vegetation structure at a fine-scale over time. In this study, we explore the comparative impacts of fire and elephant on 2,522 individually identified large trees (≥5 m in height) in the Kruger National Park, South Africa. Data were collected from 21 transects first surveyed in April 2006 and resurveyed in November 2008, to determine the relative importance of past damage by these agents on subsequent impacts and mortality. The occurrence of fire or elephant damage in 2006 affected the amount of tree volume subsequently removed by both these agents; elephant removed more tree volume from previously burned trees and the impact of subsequent fire was higher on previously burned or elephant-utilized trees than on undamaged trees. Mortality was also affected by an interaction between previous and recent damage, as the probability of mortality was highest for trees that suffered from fire or elephant utilization after being pushed over. Subsequent fire damage, but not elephant utilization, on debarked trees also increased the probability of mortality. Mortality was twice (4.6% per annum) that of trees progressing into the ≥5 m height class, suggesting an overall decline in large tree density during the 30-month study period. The responses of large trees were species and landscape-specific in terms of sensitivity to elephant and fire impacts, as well as for levels of mortality and progression into the ≥5 m height class. These results emphasize the need for fine-scale site-specific knowledge for effective landscape level understanding of savanna dynamics.     

Spectroscopical identification of residues of subunit G of the yeast V-ATPase in its connection with subunit E

A critical point in the V₁ sector and entire V₁V_O complex is the interaction of stalk subunits G (Vma10p) and E (Vma4p). Previous work, using precipitation assays, has shown that both subunits form a complex. In this work, we have analysed the N-terminal segment of subunit G (G_1–59) of the V₁V_O ATPase from Saccharomyces cerevisiae by using nuclear magnetic resonance (NMR) spectroscopy. Analyses of ¹H-¹⁵N heteronuclear single quantum coherence (HSQC) spectra of G_1–59 in the absence and presence of the N-terminal peptides E_1–18 and E_18–38 as well as the produced and purified C-terminal segment (E_39–233) shows specific interactions only with the peptide fragment E_18–38. The binding of this peptide occurs via the residues M₁, V₂, S₃, and K₅ as well for V₂₂, S₂₃, K₂₄, A₂₅ and R₂₆ of G_1–59. The specific E_18–38/G_1–59 binding has been confirmed by fluorescence correlation spectroscopy data. The E_18–38 peptide has been studied by CD spectroscopy and NMR. The 3D structure of this peptide adopts a stable helix-hinge-helix formation in solution. A model structure of the E_18–38/G_1–59 complex reveals the orientation of E_18–38 relative to G_1–59 via salt-bridges of the polar residues and van der Waals forces at the very N-terminus of both segments.