NISCAIR Online Periodicals Repository Community: IJBB Vol.37 [2000]

Oxidative stress in cucumber (Cucumis sativus L) seedlings treated with acitfluorfen

Title: Oxidative stress in cucumber (Cucumis sativus L) seedlings treated with acitfluorfen

Authors: Gupta, Illora; Tripathy, Baishnab Charan

Abstract: Treatment of diphenyl ether herbicide actifluorfen-Na (AF-Na) to intact cucumber (Cucumis sativus L cv Poinsette) seedlings induced overaccumulation of protoporphyrin IX in light (75 μmole m^-2 s^-1). The extra-plastidic protoporphyrin IX accumulated during the light exposure disappeared within two hours of transfer of acifluorofen-treated seedlings to darkness. The dark disappearance was due to re-entry of migrated protoporphyrin IX into the plastid and its subsequent conversion to protochlorophyllide. In light, protoporphyrin IX acted as a photosensitizer and caused generation of active oxygen species. The latter caused damage to the cellular membranes by peroxidation of membrane lipids that resulted in production of malondialdehyde. Damage to the plastidic membranes resulted in damage to photosystem I and photosystem II reactions. Dark-incubation of herbicide-sprayed plants before their exposure to light enhanced photodynamic damage due to diffusion of the herbicide to the site of action. Compared to control, in treated samples the cation-induced increases in variable fluorescence/maximum fluorescence ratio and increase in photosystem II activity was lower due to reduced grana stacking in herbicide-treated and light-exposed plants.

Page(s): 498-505

Carboxy terminal region of a chloroplast DNA polymeras accessory factor stimulates DNA polymerase activity

Title: Carboxy terminal region of a chloroplast DNA polymeras accessory factor stimulates DNA polymerase activity

Authors: Gaikwad, Amos; Hop, Duong Van; Mukherjee, Sunil Kumar

Abstract: p43, a glycoprotein of pea chloroplast (ct), acts as an accessory protein of pea chloroplast DNA polymerase. p43 binds to DNA, binds to ct-DNA polymerase and stimulates the ct-DA polymerase activity. In the work presented here, the C-terminal domain of p43 (p22) has been overexpressed in E. coli. South Western analysis reveals that the recombinant p22 lacks in DNA binding activity. However, the recombinant p22 can form complex with the pea ct-DNA polymerase quite efficiently and stimulates the DNA polymerase activity to a greater extent than the native p43. Thus the DNA binding domain of p43 appears to be spatially separate from the domain responsible for the DNA polymerase accessory activity. The DNA binding domain is also highly O-glycosylated and loss of glycosylation of p43 leads to enhanced DNA binding as well as repression of ct-DNA polymerase activity. These findings allow us to propose a model to explain how glycosylation of p43 helps ct-DNA polymerase latch onto the DNA template for enhanced processivity. The predictive components of the model have been discussed.

Page(s): 424-432

Low dose UV-B induced modification of chromophore conformation and it's interaction with microenvironment in cyanobacterial phycobilisomes

Title: Low dose UV-B induced modification of chromophore conformation and it's interaction with microenvironment in cyanobacterial phycobilisomes

Authors: Jha, Indra Brata; Kolli, Bala K; Sah, Jerome F; Garab, Gyozo; Mohanty, Prasanna

Abstract: Phycobilisomes (Pbsomes) are the supra macromolecular pigment protein complexes of cyanobacteria. Synechococcus Pbsomes are comprised of phycocyanins (PC) and allophycocyanins (APC).Pbsomes are major light harvesting antennae and also absorb ultraviolet-B (UV-B) radiation (280-320 nm). Synechococcus Pbsomes, upon exposure to low dose of UV-B (0.28 mW cm^-2) for different time intervals showed profound alteration in their steady state absorption, fluorescence excitation and emission characteristics (Sah et. al. Biochem. Mol. Biol.Int., Vol. 44, No.2, 245-247). In the present study, we investigated the effect of low dose of UV -B on isolated Pbsome of Synechococus. Our results demonstrate the following alterations. Absorbance at 623 nm initially showed a sharp decrease with increasing exposure time to UV-B radiation. The changes in the visible to near ultraviolet absorption and excitation ratio indicated a change in chromophore conformation, upon prolonged exposure of Pbsomes to UV-B radiation. This modification of chromophore conformation appeared to be associated with the loss of energy transfer from PC to APC. Circular dichroism spectra in the amide region showed a significant loss of the α helical content of Pbsomes when exposed for longer duration to UV-B. CD spectra in the visible region revealed a marked decrease in the rotational strength at 620 nm. Close monitoring of CD signals emanating in the 500 to 700 nm range further revealed that the decrease in the rotational strength was closely associated with an initial red shift in the positive CD band of Pbsomes when exposed to UV -B for short duoration. However, the peak became constant over prolonged exposure to UV-B radiation and accompanied a prominent blue shoulder in the positive CD band which suggests the modification and uncoupling of the various phycocyanobilin (PCB) chromophores of the Synechococcus Pbsomes.

Page(s): 486-490

Biogenesis and assembly of photosystem I

Title: Biogenesis and assembly of photosystem I

Authors: Schwabe, Tatjana M E; Kruip, Jochen

Abstract: Photosystem I (PS I) is a multi subunit membrane protein complex consisting of 11 to 14 different subunits. In addition, several cofactors, such as chlorophylls, phylloquinones, carotenoids and iron-sulfur clusters are bound by this complex. We now have a detailed understanding of the structural basics, yet we know very little about the molecular details of the assembly process that finally yields functional PS I. Moreover, not much is known about the molecular dynamics of PS I in the thylakoid membrane or its regulated degradation. These areas have become the focus of recent work and first results have emerged. In this minireview we describe the latest findings in this fascinating and rapidly evolving field.

Page(s): 351-359