EPR study of the oxygen evolving complex in His-tagged photosystem II fromthe cyanobacterium Synechococcus elongatus

The Mn-4-cluster and the cytochrome c(550) in histidine-tagged photosystem II (PSII) from Synechococcus elongatus were studied using electron paramagn etic resonance (EPR) spectroscopy. The EPR signals associated with the S-0- state (spin 1/2) and the S-2-state (spin 1/2 and IR-induced spin 5/2 state) were essentially identical to those detected in the non-His-tagged strain. The EPR signals from the S-3-state, not previously reported in cyanobacter ia, were detectable both using perpendicular (at g = 10) and parallel (at g = 14) polarization EPR, and these signals are similar to those found in pl ant PSII. In the Ss-state, near-infrared illumination at 50 K induced a 176 -G-wide split signal at a = 2 and signals at g = 5.20 and g = 1.51. These s ignals differ slightly from those reported in plant PSII [Ioannidis, N,, an d Petrouleas, V. (2000) Biochemistry 39, 5246-5254]. In accordance with the cited work, the split signal presumably reflects a radical interacting wit h the Mn-4-cluster in a fraction of centers, while the g = 5.20 and g 1.51 signals are tentatively attributed to a high-spin state of the Mn-4-cluster with zero field splitting parameters different from those in plant PSII, r eflecting minor changes in the environment of the Mn-4-cluster. Biochemical modifications (Sr2+/Ca2+ substitution, acetate and NH3 treatments) were al so investigated. In Sr2+-reconstituted PSII, in addition to the expected mo dified S-2 multiline signal, a signal at g = 5.2 was present instead of the g approximate to 4 signal seen in plant PSII, In NH3-treated samples, in a ddition to the expected modified S-2-multiline signal, a g approximate to 4 signal was detected in a small proportion of the reaction centers. This is of note since g approximate to 4 spectra arising from the Mn-4-cluster in the S-2 state have not yet been published in cyanobacterial PSII, The detec tion of modified S-3-signals in both perpendicular (at g = 7.5) and paralle l (at g = 12) polarization EPR from NH3-treated PSII indicate that NH3 is s till bound in the S-3-state. The acetate-treated PSII behaves essentially a s in plant PSII, A study using oriented samples indicated that the heme pla ne of the oxidized low spin Cytc(550) was perpendicular to the plane of the membrane.