Cytochrome c-550, a low-potential c-type cytochrome, and a 12-kDa protein were recently shown to be associated extrinsically and stoichiometrically with purified photosystem II (PSII) complex of the thermophilic cyanobacterium Synechococcus vulcanus [Shen, J.-R., Ikeuchi, M., & Inoue, Y. (1992) FEBS Lett. 301, 145-149]. The binding and functional properties of these two extrinsic components in PSII were studied by means of release-reconstitution and thermoluminescence techniques. The following results were obtained: (i) cyt c-550 rebound appreciably to cyanobacterial PSII in the absence of the 33-and 12-kDa extrinsic proteins, but the presence of these two proteins facilitated the rebinding, affording a full level of binding equal to that in native PSII. (ii) The 12-kDa protein did not rebind to PSII at all unless the 33-kDa protein or cyt c-550 was present. It rebound only partially in the presence of either of these two proteins, but it rebound maximally when reconstituted together with both of them. (iii) Reconstitution with cyt c-550 or the 12-kDa protein alone in the absence of the 33-kDa protein did not restore the O2-evolving activity of CaCl2-washed PSII. Reconstitution with cyt c-550 in combination with the 33-kDa protein appreciably enhanced the activity, but the activity restoration was much more marked and reached a level close to that of the original activity when all three extrinsic proteins were included. (vi) Analysis of the light intensity dependence of O2 evolution and thermoluminescence glow curves revealed that cyt c-550 regulates the efficiency of S-state transition between S1 and S2 or S2 and S3 or both, whereas the 12-kDa protein modulates a dark step(s) in O2 evolution. It was inferred from these results that the two new cyanobacterial extrinsic components, cyt c-550 and the 12-kDa protein, are closely dependent on each other and also on the extrinsic 33-kDa protein with respect to their binding and functional properties. They bind to the donor side of cyanobacterial PSII and play important roles in regulating the O2-evolving activity.
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