[Kinetics of dark reduction of P700 centers in photosynthesis]

Relationship between the rate of dark fall of ESR I signal in the chloroplasts and light intensity 710 nm was studied. It is shown that the kinetics of the signal drop at sufficiently high levels of intensity is a biphase one. The fast phase (2 sec) disappears in the presence of an inhibitor of noncyclic electron transfer. It is concluded that the kinetics of electron transport is not described within the concept of the linear chain of electron carriers and a suggestion is put forward concerning a possible mechanism of this process which depends on the participation of plastoquinone in electron transport.

[Cytochrome b-559 photooxidation in the presence of carbonyl cyanide p-trifluorometh-oxyphenylhydrazone and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or p-benzoquinone in three non-photosynthetic mutants of Chlamydomonas reinhardti (author's transl)]

Studies of absorbance related to the cytochrome b-559 photooxidation induced by FCCP, with and without addition of 3-p-chlorophenyl-1, 1-dimethylurea (CMU), DBMIB or p-benzoquinone, in whole cells and in chloroplast fragments of Chlamydomonas reinhardti, were carried out. In addition to the wild type, three strains of non-photosynthetic mutants were used: Fl 5, which lacks P 700; Fl 9 and Fl 15, which are deficient in bound cytochrome c-553 and in cytochrome b-563. In the presence of FCCP, whole cells and chloroplast fragments of the four strains showed a System II-dependent photooxidation of cytochrome b-559. This photooxidation was inhibited by CMU but it occurred again in presence of FCCP, CMU and DBMIB. In chloroplast fragments, cytochrome b-559 photooxidation was also inhibited by an excess of FCCP; it was recovered, likewise, by addition of DBMIB. In whole cells, the highest measured redox changes were: 1 mu mol oxidized cytochrome b-559 per 1 mmol chlorophyll, corresponding approximately to about one seventh (wild type, Fl5) or one fifth (Fl 9, Fl 15) of the total amount of this cytochrome. Another kind of cytochrome b-559 photooxidation, CMU-insensitive, also occurred in the mutants Fl 9 and Fl 15 and in the wild type, but not in the mutant Fl 5. This latter kind of photooxidation was observed with chloroplast fragments in the presence of FCCP and CMU and also with whole cells in the presence of FCCP, CMU and p-benzoquinone. These reactions can be attributed to the Photosystem I; they do not require the intervention of the cytochrome c-553. A high-potential form of cytochrome b-559, hydroquinone-reducible, was involved in these two kinds of photooxidation. In addition, a lower potential form, reducible only by ascorbate, appeared to be able to interfere also. An interpretation is attempted, taking into consideration the various effects of FCCP and DBMIB, at different concentrations, on photosynthetic electron transport.

The effect of inhibitors on the electron flow triggering photo-phobic reactions in Cyanophyceae

1. Since photo-phobic reactions in the blue green alga Phormidium uncinatum seem to be triggered by changes of electron flow rates into or out of an electron pool situated in the electron transport chain between photo-system II and I, the effect of inhibitors affecting the electron transport chain has been studied. 2. Dose response curves of the phobic reaction have been measured by varying the trap energy in double beam light trap experiments with constant pairs of monochromatic light. From these dose response curves the effects of the inhibitors on both types of phobic reactions, i.e. exit reactions and entrance reactions, have been calculated. 3. Dibromothymoquinone (DBMIB) inhibits the electron transport between the electron pool and photosystem I by preventing the reoxidation of plastoquinone. The phobic entrance reaction, which results in an emptying of the light trap, is triggered by changes in the electron flow out of the pool; thus it is more effected by DBMIB than the exit reaction, which is mediated by the electron transport into the pool. 4. The phobic exit reaction, which results in accumulations in the light trap, is triggered by changes in the electron flow into the electron pool via photosystem II. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibits the electron transport near photosystem II; thus it affects the exit reaction more than the entrance reaction.

Interactive mutagenicity of sodium nitrite, dimethylamine, methylurea and ethylurea

Groups of mice were treated per os with sodium nitrite either alone or in combination with nitrosatable amino compounds and tested in the host mediated assay. When mice were treated with sodium nitrite in combination with dimethylamine a small(4-fold) but significant increase in mutant frequency (MF) was observed. Ethylurea or methylurea in combination with sodium nitrite induced 10- or 850-fold increases in MF, respectively. The response to methylurea was dose-dependent with a 6- and 30-fold increase in MF at 5.4 and 11.5 mg/kg NaNO2 and a 6-fold increase at 108 mg/kg methylurea. That this response reflected gastric nitrosation was shown by the disappearance of the response if NaNO2 administration preceded methylurea treatment by 10 min. High MF's were observed if NaNO2 was administered 10 or 20 min after methylurea.