Evidence for a biological role in photosynthesis for cytochrome b-559--a component of photosystem II reaction center.
Absorbance changes in untreated intact leaf discs, produced upon excitation with high-intensity red light, were shown to be due to the photooxidation of cytochrome b-559. At low intensities (<100 W/m2), photooxidation was almost undetectable. Photooxidation occurred with a half-time of 4.3 sec and an extent of 0.64 mol of cytochrome per 320 mol of chlorophyll. Upon transition to darkness, an additional oxidation occurred that exhibited faster kinetics (t/12 < 100 msec) and 0.32 mol of cytochrome was oxidized per 320 mol of chlorophyll. Photooxidation was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and was specifically induced by red light since far-red light did not cause any absorbance decrease. These results suggest that the redox changes of cytochrome b-559 are driven by photosystem II. Photooxidation was increased by 67% and its initial rate was doubled upon incubation of the leaf in carbonylcyanide p-trifluoromethoxy-phenylhydrazone. Exposure of the leaf to mild water stress or mild heat stress resulted in an increase in the extent of photooxidation and in a 6-fold decrease in the rate constant. Mild heat stress also induced a large increase of the rate constant for the dark reduction of the cytochrome. The dependence of photooxidation on high-intensity red light, its inhibition criteria, the fast transient dark oxidation, and enhancement of both photooxidation and dark transient oxidation by treatments that affect Z, the primary donor to P680, indicate that cytochrome b-559 in vivo is involved in cyclic electron flow around photosystem II. Its primary role in photosynthesis is to divert excess photons from a linear to a cyclic electron flow at high light intensities for protection of the D1 and D2 proteins against photodamage. Dark oxidation of the cytochrome is suggested to reflect a second role, that of deactivation of the powerful oxidant Z+ in the dark.
Canaani
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O
Biochemistry Department, Weizmann Institute of Science, Rehovot, Israel.
Havaux
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