FUNCTIONAL SIZE OF PHOTOSYSTEM-II DETERMINED BY RADIATION INACTIVATION

The functional size of Photosystem II (PS II) was investigated by radi ation inactivation. The technique provides an estimate of the function al mass required for a specific reaction and depends on irradiating sa mples with high energy gamma-rays and assaying the remaining activity. The analysis is based on target theory that has been modified to take into account the temperature dependence of radiation inactivation of proteins. Using PS II enriched membranes isolated from spinach we dete rmined the functional size of primary charge separation coupled to wat er oxidation and quinone reduction at the Q(B) site: H2O --> (Mn)(4) - -> Y-2 --> P680 --> Pheophytin --> Q --> phenyl-p-benzoquinone. Radiat ion inactivation analysis indicates a functional mass of 88 +/- 12 kDa for electron transfer from water to phenyl-p-benzoquinone. It is like ly that the reaction center heterodimer polypeptides, D1 and D2, contr ibute approximately 70 kDa to the functional mass, in which case polyp eptides adding up to approximately 20 kDa remain to be identified. Lik ely candidates are the alpha and beta subunits of cytochrome b(559) an d the 4.5 kDA psbI gene product.