Correlation of the cytochrome c(550) content of cyanobacterial Photosystem II with the EPR properties of the oxygen-evolving complex

The Mn-4 cluster of PS II advances through a series of oxidation states (S states) that catalyze the breakdown of water to dioxygen in the oxygen-evolving complex. The present study describes the engineering and purification of highly active PS II complexes from mesophilic His-tagged Synechocystis PCC 6803 and purification of PS II core complexes from thermophilic wild-type Synechococcus lividus with high levels of the extrinsic polypeptide, cytochrome c(550). The g = 4.1 S-2 state EPR signal, previously not characterized in untreated cyanobacterial PS II, is detected in high yields in these PS II preparations. We present a complete characterization of the g = 4.1 state in cyanobacterial His-tagged Synechocystis PCC 6803 PS II and S. lividus PS II. Also presented are a determination of the stoichiometry of cytochrome c(550) bound to His-tagged Synechocystis PCC 6803 PS II and analytical ultracentrifugation results which indicate that cytochrome c(550) is a monomer in solution. The temperature-dependent multiline to g = 4. 1 EPR signal conversion observed for the S-2 state in cyanobacterial PS II with high cytochrome c(550) content is very similar to that previously found for spinach PS II. In spinach PS II, the formation of the S-2 state g = 4.1 EPR signal has been found to correlate with the binding of the extrinsic 17 and 23 kDa polypeptides. The finding of a similar correlation in cyanobacterial PS II with the binding of cytochrome c(550) suggests a functional homology between cytochrome c(550) and the 17 and 23 kDa extrinsic proteins of spinach PS II.