Light and dark rate-determining steps in electron transport reactions in spinach chloroplasts

Effects of various factors, such as uncouplers, inhibitors andinhibitory treatments of chloroplasts, on light-dependent (K_L)and -independent (K_D) parameters estimated from the light intensity-activityrelationship, were studied. In the Hill-reaction with ferricyanide or methyl viologen aselectron acceptor, a reagent or treatment affecting electrontransport on one side of system II, which included the rate-limitingstep for the entire electron transport, affected only K_D. Incontrast, a change in the rate of electron transfer on the otherside of system II affected only K_L. We inferred that K_D representsthe rate of the dark rate-limiting step at infinite light intensity.On the other hand, K_L is concerned to the quantum yield of theprimary reaction, as well as to the rate constant of the reactionon the side of system II opposite to that of the rate-limitingstep at infinite light intensity. Effects of inhibitors and treatments on the reaction parameterschanged markedly depending on the pH of the reaction medium.However, the contrasting effects of inhibitors affecting theopposite sides of system II were consistently observed in definedlevels of pH. This was also the case in the photoreduction ofmethyl viologen with the ascorbate-DPIP couple as electron donor. ¹ Present address: Department of Biology, Faculty of Science,Toho University, Narashino, Chiba, Japan. (Received May 2, 1972; )     

Distinct pathways for jasmonate- and elicitor-induced expressions of a cytochrome P450 gene in soybean suspension-cultured cells

The mechanisms of the jasmonate-induced expression of genes encoding the cytochrome P450 CYP93A1 and lipoxygenase L-4 were analyzed in a soybean photomixotrophic cultured cell line, SB-P. The induction of the cytochrome P450 gene caused by methyl jasmonate (MeJA) was specifically suppressed by trifluoperazine and DCMU, inhibitors of chloroplast electron transport. Additionally, induction of the cytochrome P450 gene required irradiation. In contrast, induction of the lipoxygenase L-4 gene by the MeJA treatment occurred in both dark and light. Based on the results, the presence of two distinct signalling pathways for jasmonate-inducible gene expression, light-dependent and light-independent, is proposed. The jasmonate-inducible cytochrome P450 was also specifically induced by a fungal elicitor from a cell wall fraction of Phytophthora megasperma , a fungal pathogen, suggesting a role for P450 in the defense response to fungus in soybean cells. However, trifluoperazine did not block the elicitor-induced expression of cytochrome P450.     

A new light-induced absorbance change at 561 nm in chloroplasts of green alga, Bryopsis maxima

A new light-induced absorbance change having a maximum at 561nm was discovered in the thalli, as well as in isolated chloroplastsof a green alga, Bryopsis maxima Okamura. Another simultaneous change also occurred at 515 nm. The magnitudeof the 561 nm change was several-fold larger than that at 515nm and much larger than could be explained by an oxidation-reductionchange in cytochromes contained in chloroplasts. There was noabsorbance change in the Soret region that may be correlatedto the 561 nm change. Both 561 and 515 nm changes showed a spike-liketime course pattern, both having a half-rise time of about 20msec. Effects of inhibitors and uncouplers such as DCMU, Cl-CCPand gramicidin J on the absorbance change were also similarat 561 and at 515 nm. We inferred that the 561 nm change is related to photophosphorylationand possibly to the membrane potential in a way similar to the515 nm change. (Received March 27, 1974; )     

Isolation and properties of membrane-bound cytochrome c-552 from photosynthetic bacterium Chromatium vinosum

A membrane-bound cytochrome c-552 was isolated and purified from the photosynthetic bacterium Chromatium vinosum by treatment with sodium cholate, sodium deoxycholate and bacterial alkaline protease followed by gel filtration. The purified cytochrome c-552, which may have been modified by the protease treatment, was electrophoretically homogeneous. Its minimal molecular weight was estimated to be 19 and 20 kdaltons, respectively by SDS polyacrylamide gel electrophoresis and by gel filtration on Sephadex G-100. Cytochrome c-552 showed the absorption maxima at 419, 523 and 552 nm in the reduced form. Reduced-minus-oxidized difference millimolar absorption coefficient was 10.6 for the wavelength pair, 552 minus 540 nm. The midpoint potential at pH 8.0 was ?130 mV. The polarity in the amino acid composition of cytochrome c-552 was 40.1% and reflected its hydrophobicity. The solubilized cytochrome c-552 was shown to be a different entity from the soluble flavocytochrome c-552 in several respects.