Abstract
It is shown here that a dual bicarbonate-reversible formate inhibition of electron flow exists in the intact cells of a green eukaryotic alga,
Chlamydomonas reinhardtii. There are two suggested sites of inhibition, one after and the other before Q
A, where Q
A is the bound plastoquinone electron acceptor of Photosystem II (PS II). A long term (hours) formate treatment slows the rate of oxidation of Q
A-, measured by variable chlorophyll
a fluorescence decay after an actinic flash. Concomitant with a 2-fold increase in amplitude of the slow component, the amplitude of the fast component decreases at both pH 6.5 and 7.5. The half-times of faster decay increase from 285 µs to 560 µs (pH 6.5) and from 256 µs to 490 µs (pH 7.5), and the slower decay from 37 to 60 ms (pH 6.5) and from 24 to 80 ms (pH 7.5). Addition of 2.5 mM HCO
3 — to the formate-treated samples fully reverses all these effects. The anion effects on the electron flow from Q
A — to Q
B (where Q
B is the second plastoquinone acceptor) were accompanied by effects on the dimethylquinone/ferricyanide Hill reaction: a 4-fold stimulation was observed upon addition of 20 mM HCO
3 — (pH adjusted, 6.5) to formate-treated samples. Since this stimulation was observed in the presence of 2,5-dibromo-6-isopropyl-p-benzoquinone, which inhibits intersystem electron flow, it is unrelated to CO
2 fixation in intact systems. A short term (minutes) formate treatment reversibly diminishes chlorophyll
a fluorescence. This effect is present even in mild heat-treated (45 °C, 3 min) and 10 mM hydroxylamine-treated (pH 7.3) cells. Thus, it is suggested that it is between the hydroxylamine donation site (electron donor Z or D) and Q
A.