EFFECT OF NEAR-INFRARED LIGHT ON THE S-2-STATE OF THE MANGANESE COMPLEX OF PHOTOSYSTEM-II FROM SYNECHOCOCCUS-ELONGATUS

The Mn cluster of Photosystem II (PSII) from Synechococcus elongatus w as studied using EPR. A signal with features between g = 5 and g = 9 i s reported from the S-2-state. The signal is attributed to the mangane se cluster in a state with a spin 5/2 state. Spectral simulations of t he signal indicate zero field splitting parameters where the \E/D\ was 0.13. The new signal is formed by irradiating PSII samples which cont ain the spin 1/2 S-2-state using 813 nm light below 200 K. This effect is attributed to a spin-state change in the manganese cluster due to absorption of the IR light by the Mn-cluster itself. The signal is sim ilar to that reported recently in PSII of plants [Boussac, A., tin, S. , Horner, 0., and Rutherford, A. W, (1998) Biochemistry 37, 4001-4007] . In plant PSII the comparable signal is formed at a lower temperature (optimally below 77 K), and gradual warming of the sample in the dark leads to the formation of the state responsible for the well-known g = 4.1 signal prior to formation of the spin 1/2 multiline signal. In t he present work using cyanobacterial PSII, warming of the sample in th e dark leads to the formation of the spin 1/2 multiline signal without formation of the g 4 type signal as an intermediate. These observatio ns provide a partial explanation for the long-standing ''mystery of th e missing g = 4 state'' in cyanobacterial PSII, The observations are r ationalized in terms of three possible states which can exist for S-2: (i) the spin 1/2 multiline signal, (ii) the state responsible for the g = 4.1 signal, and (iii) the new spin 5/2 state. The relative stabil ity of these states differs between plants and cyanobacteria.