EPR signal II in cyanobacterial Photosystem II reaction-center complexes with and without the 40 kDa chlorophyll-binding subunit

The steady-state amplitude and flash-induced kinetics of EPR signal II in two Photosystem II (PS II) reaction center protein complexes from Synechococcus were measured to probe the organization of species involved in the PS II electron-transfer chain. A PS II reaction center complex (E-1) which has 47, 40, 31, 28 and 9 kDa subunits shows both fast decaying (signal II_f) and slowly decaying (signal II_s+u) EPR components. The amplitude of signal II_f, which represents Z (the donor to P-680), is about 1 spin per 30 Chl. This corresponds to one spin per reaction center in this preparation. Signal II_s+u, the slowly decaying component of signal II, reflects D, a donor to PS II on a side chain from the path of water oxidation in higher plants and algae. Signal II_s+u is present in the E-1 preparation in a ratio of about 1 spin per 40 Chl. Flash-induced signal II_f in E-1 shows biexponential decay with half-times of 20 ms and 300 ms. In a PS II reaction center complex (CP2b) which has 47, 31, 28 and 9 kDa subunits, but no 40 kDa subunit, an appreciable amount of signal II_f is observed (about 1 per 50 Chl). Less than 1 spin per 400 Chl of signal II_s+u is visible in this sample. The kinetics of Z⁺ reduction (signal II_f) in CP2b is similar to that seen in E-1 preparations, indicating that CP2b contains all of the molecules necessary for primary charge separation and secondary electron donation from Z.