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We evaluate the protective effect of nitric oxide (NO) against Cadmium (Cd) toxicity in rice leaves. Cd toxicity of rice leaves was determined by the decrease of chlorophyll and protein contents. CdCl 2 treatment resulted in (1) increase in Cd content, (2) induction of Cd toxicity, (3) increase in H 2 O 2 and malondialdehyde (MDA) contents, (4) decrease in reduced form glutathione (GSH) and ascorbic acid (ASC) contents, and (5) increase in the specific activities of antioxidant enzymes (superoxide dismutase, glutathione reductase, ascorbate peroxidase, catalase, and peroxidase). NO donors [N-tert-butyl--phenylnitrone, 3-morpholinosydonimine, sodium nitroprusside (SNP), and ASC + NaNO 2 ] were effective in reducing CdCl 2 -induced toxicity and CdCl 2 -increased MDA content. SNP prevented CdCl 2 -induced increase in the contents of H 2 O 2 and MDA, decrease in the contents of GSH and ASC, and increase in the specific activities of antioxidant enzymes. SNP also prevented CdCl 2 -induced accumulation of NH 4 + , decrease in the activity of glutamine synthetase (GS), and increase in the specific activity of phenylalanine ammonia-lyase (PAL). The protective effect of SNP on CdCl 2 -induced toxicity, CdCl 2 -increased H 2 O 2 , NH 4 + , and MDA contents, CdCl 2decreased GSH and ASC, CdCl 2 -increased specific activities of antioxidant enzymes and PAL, and CdCl 2 -decreased activity of GS were reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, a NO scavenger, suggesting that protective effect by SNP is attributable to NO released. Reduction of CdCl 2 -induced toxicity by NO in rice leaves is most likely mediated through its ability to scavenge active oxygen species including H 2 O 2 .

The electron-attachment-induced proton transfer in the guanine-cytosine (G:C) base pair is thought to be relevant to the issues of charge transport and radiation damage in DNA. However, our understanding on the reaction mainly comes from the data of isolated bases and base pairs, and the behavior of the reaction in the DNA duplex is not clear. In the present study, the proton-transfer reaction in reduced G:C stacks is investigated by quantum mechanical calculations with the aim to clarify how each environmental factor affects the proton transfer in G:C(*-). The calculations show that while the proton transfer in isolated G:C(*-) is exothermic with a small energetic barrier, it becomes endothermic with a considerably enhanced energetic barrier in G:C stacks. The substantial effect of G:C stacking is proved to originate from the electrostatic interactions between the dipole moments of outer G:C base pairs and the middle G:C(*-) base-pair radical anion; the extent of charge delocalization is very small and plays little role in affecting the proton transfer in G:C(*-). On the basis of the electrostatic model, the sequence dependence of the proton transfer in the ionized G:C base pair is predicted. In addition, the water molecules in the first hydration shell around G:C(*-) display a pronounced effect that facilitates the proton-transfer reaction; further consideration of bulk hydration only slightly lowers the energetic barrier and reaction energy. We also notice that the water arrangement around an embedded G:C(*-) is different from that around an isolated G:C(*-), which could result in a very different solvent effect on the energetics of the proton transfer. In contrast to the important influences of base stacking and hydration, the effects of sugar-phosphate backbone and counterions are found to be minor. Our calculations also reveal that a G:C base pair embedded in DNA is capable of accommodating two excess electrons only in bulk hydration; the resultant G(N1-H)(-):C(N3+H)(-) dianion is stable and exists long enough to lead to DNA damage. The combination of the present results with the previous findings in literature suggests that the behaviors of charge transport and low-energy electron-induced damage in DNA are highly susceptible to the hydration level.

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In vitro catalytic activity of DesVII, the glycosyltransferase involved in the biosynthesis of methymycin, neomethymycin, narbomycin, and pikromycin in Streptomyces venezuelae, is described. This is the first report of demonstrated in vitro activity of a glycosyltransferase involved in the biosynthesis of macrolide antibiotics. DesVII is unique among glycosyltransferases in that it requires an additional protein component, DesVIII, as well as basic pH for its full activity.

The present study examined the response of antioxidant systems to NaCl stress and the relative importance of Na 1 and Clin NaCl-induced antioxidant systems in roots of rice seedlings. NaCl treatment caused an increase in the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) in roots of rice seedlings, but had no effect on the activities of superoxide dismutase (SOD) and catalase (CAT). There were detectable differences in APX and GR isoenzymes between control and NaCl-treated roots. Levels of activity for SOD and CAT isoenzymes did not change in NaCl-stressed roots compared with the control roots. NaCl treatment produced an increase in H 2 O 2 , ascorbate (AsA), dehydro-ascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) levels. Treatment with 50 mM Na-gluconate (whose anion is not permeable to membrane) led to a similar Na 1 level in roots to that with 100 mM NaCl. It was found that treatment with 50 mM Na-gluconate affected H 2 O 2 , AsA, and DHA levels, APX and GR activities, OsAPX and OsGR mRNA induction in the same way as 100 mM NaCl. These observed changes seem to be mediated by Na 1 toxicity and not by Cltoxicity. On the other hand, it was found that NaCl, but not Na-gluconate and NaNO 3 , caused an increase in GSH and GSSG levels, indicating that Cl -, rather than Na 1 , is responsible for the NaCl-increased GSH and GSSG levels in roots of rice seedlings.

The mechanism of light-inhibited ethylene production in excised rice (Oryza sativa L.) and tobacco (Nicotiana tabacum L.) leaves was examined. In segments of rice leaves light substantially inhibited the endogenous ethylene production, but when CO2 was added into the incubation flask, the rate of endogenous ethylene production in the light increased markedly, to a level which was even higher than that produced in the dark. Carbon dioxide, however, had no appreciable effect of leaf segments incubated in the dark. The endogenous level of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, was not significantly affected by lightdark or CO2 treatment, indicating that dark treatment or CO2exerted its effect by promoting the conversion of ACC to ethylene. This conclusion was supported by the observations that the rate of conversion of exogenously applied ACC to ethylene was similarly inhibited by light, and this inhibition was relieved in the presence of CO2. Similar results were obtained with tobacco leaf discs. The concentrations of CO2 giving half-maximal activity was about 0.06%, which was only slightly above the ambient level of 0.03%. The modulation of ACC conversion to ethylene by CO2 or light in detached leaves of both rice and tobacco was rapid and fully reversible, indicating that CO2 regulates the activity, but not the synthesis, of the enzyme converting ACC to ethylene. Our results indicate that light inhibition of ethylene production in detached leaves is mediated through the internal level of CO2, which directly modulates the activity of the enzyme converting ACC to ethylene.