NO reversibly reduces the water-oxidizing complex of photosystem II through S-0 and S-1 to the state characterized by the Mn(II)-Mn(III) multiline EPR signal

Incubation of photosystem II preparations with NO at -30 degrees C results in the slow formation of a unique state of the water-oxidizing complex (WOC ), which was recently identified as a Mn(II)-Mn(III) dimer [Sarrou, J., Ioa nnidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3 581-3587]. Evolution of the Mn(II)-Mn(III) EPR signal proceeds through one or more intermediates [Goussias, C., Ioannidis, N., and Petrouleas, V. (199 7) Biochemistry 36, 9261-9266]. In an effort to identify these intermediate s, we have examined the time course of the signal evolution in the presence and absence of methanol. An early step of the interaction of NO with the W OC is the reduction of S-1 to the S-0 state, characterized by the weak Mn-h yperfine structure recently reported for that state. At longer times S-0 is further reduced to a state which has the properties of the S-1 state, in t hat single-turnover illumination restores the S-0 signal. The Mn(II)-Mn(III ) state forms after the S-1 state and is tentatively assigned to an (iso)S- 2 state, although lower states or a modified S-1 state cannot. be excluded at present. Following removal of NO 60-65% of the initial S-2 multiline sig nal size or the O-2-evolving activity can be restored. The data provide for the first time EPR information on a state lower than S-0. Furthermore, the low-temperature NO treatment provides a simple means for the selective pop ulation of the S-0, S-1 and the Mn(II)-Mn(III) states.