Biochimica et Biophysica Acta (BBA) - Biophysics including Photosynthesis最新文献

The role played by the nucleus in the circadian rhythms in Acetabularia mediterranea was investigated: these rhythms were in photosynthetic capacity and in chloroplast shape. The following results were obtained.

• 1.

  1. Anucleate Acetabularia are able to maintain their rhythm of photosynthetvc capacity and to restore this rhythm after it has been abolished by physical means.

• 2.

  2. The same conclusion is true for the rhythm in chloroplast shape.

• 3.

  3. Actinomycin D dramatically inhibits the rhythm in photosynthetic capacity in intact algae. The rhythm is strongly decreased within 6 days by a concentration of 2.7 μg/ml; at a concentration of 0.27 μg/ml for 12–16 days the rhythm is abolished or very strongly reduced, depending on the experiment.

• 4.

  4. Actinomycin D also prevents the rhythmic variation in chloroplast shape with the time period of the cycle, at the same concentration and for the same length of treatment as used for the inhibition of the rhythm in photosynthetic capacity.

• 5.

  5. Actinomycin D does not affect the rhythm in photosynthetic capacity in anucleate algae at the concentrations used in the experiments with intact Acetabularia.

• 6.

  6. No difference could be detected in the behaviour of anucleate basal fragments and anucleate apical fragments in the presence of actinomycin D.

• 7.

  7. Actinomycin D does not affect the rhythm of variation in shape in chloroplasts in anucleate algae.

• 8.

  8. The two circadian rhythms, affecting photosynthetic capacity and chloroplast shape, were always correlated.

• 9.

  9. Ribonuclease induces an increase in the rhythm of photosynthetic capacity when the nucleated algae are returned to normal sea-water; on the contrary, the rhythm is not restored with photosynthesis when the treated algae are anucleate.

The results are discussed on a molecular basis and a tentative interpretation is proposed, compatible with the present experiments and with results in the literature.

• 1.

  1. A quantitative analysis of the fluorescence induction in isolated chloroplasts is presented. The rise of the fluorescence yield with time is ascribed to the photoreduction of a primary oxidant “Q” of photosystem II. The analysis yields a method of determining the amount of light actually utilized in the process. By comparing results obtained in the absence or presence of Hill oxidants, the quantum yield of results obtained in the absence or presence of Hill oxidants, the quantum yield of the primary photoreduction and the concentrations of all internal oxidants were estimated. The average value of the yield was 0.5 equiv/Einstein (for green light, 510–640 mμ). The total oxidant concentration was 1 equiv per 35 moles chlorophyll.

• 2.

  2. The shape of the rise curve as well as its intensity and temperature dependence suggest that, besides Q, a second electron carrier, P, is involved, reacting in a consecutive dark step:

  Q and P are present in a ratio 1:1, the concentration of each being 1:70 chlorophyll.

• 3.

  3. The restoration of the fluorecsence induction, i.e. the reoxidation of Q and P in the dark, which is accelerated by far-red light (700–740 mμ), was also subjected to a quantitative analysis. The quantum yield of the far-red reaction proved to be close to 1. The maximum (saturation) rate of the far-red effect was about I/50 of the Hill-reaction saturation rate.

  The results are discussed in terms of two photosystems and electron carriers arranged between them.

• 1.

  1. Fluorescence induction curves (intensity versus time) obtained by irradiation of isolated chloroplasts were observed and subjected to a detailed kinetic analysis. This analysis was based on a photosystem II model consisting of independent units, each of which contains one pigment aggregate connected to one electron-transport chain. During irradiation, two electron carriers, which exist initially in the oxidized state, are reduced in a light reaction followed by a dark reaction, both being first order:

  , where Q is the primary oxidant of photosystem II. Q and P are present in a 1:1 ratio¹. The experimental curves agree with the calculated ones over a wide range of light intensity and temperature.

• 2.

  2. Using the analysis outlined above, it was possible to estimate the first-order rate constant of the reaction between Q⁻ and P as $\text{k = 30–40}\text{sec}{}^{\mathrm{-1}}\text{at}\text{22°}\text{and}\text{2–2.5}\text{sec}{}^{\mathrm{-1}}\text{at}\text{0°}$. Comparison with the saturation rate of the Hill reaction shows that the above reaction between Q⁻ and P may be rate limiting in the Hill reaction.

The depolarization of the single-cell preparation of the electroplax of Elecrophorus electricus by acetylcholine, carbamylcholine, and trimethylbutylammonium ion is inhibited by prior treatment of the electroplax with $\text{p-}\text{chloromercuribenzoate}$ (PCMB) or with 1,4-dithiothreitol (DTT). The inhibition by either agent is characterized by an increase in the concentration of carbamylcholine eliciting a half-maximal response with no great change in the maximum response. The inhibition due to PCMB is reversed by subsequent treatment with thiol compounds, and that due to DTT is reversed by oxidizing agents and by cysteine (but not by other thiol compounds). Washing alone has no effect on the inhibition due to either agent. Treatment of the electroplax with $\text{N-}\text{ethylmaleimide}$ (NEM) after DTT blocks the reversal of the inhibition by oxidizing agents and by cysteine, added subsequently. NEM alone under the same conditions has no direct effect on the depolarization. It is concluded that PCMB is reacting with sulfhydryl groups and DTT with disulfide bonds, present in one or more component of the acetylcholine-activated permeability system at the junctional regions of the innervated membrane of the electroplax. The evidence is taken as support for the view that some of the components of this system are proteins.