Abstract
Selective light-induced redox changes of cytochrome b-559 in relation to cytochrome f have been investigated at 20°C in fresh spinach thylakoids under physiological conditions and in the presence of specific inhibitors and uncouplers. Light absorbed by PS II (650 nm) promotes the sequential reduction of cytochrome b-559 and cytochrome f, whereas light absorbed by PS I (720 nm) brings about the oxidation of cytochrome f and, by way of this, the oxidation of cytochrome b-559. Either DCMU or DBMIB completely blocks the reduction of the two cytochromes by PS II light, but not the oxidation of cytochrome f by PS I light. On the other hand, cyanide as well as polylysine inhibits almost totally the oxidation of both cytochromes f by PSI light. From these and previous results, it is concluded that photoreduction of cytochrome b-559 by PS II occurs through the plastoquinone pool and that its photooxidation by PS I involves both plastocyanin and cytochrome f. It thus appears that cytochrome b-559 is located between plastoquinone and cytochrome f in noncyclic electron transport in chloroplasts.
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