Radiation inactivation studies on Photosystem I. Functional sizes of electron-transport reactions

Radiation inactivation studies on the functional size of electron-transport processes in the Photosystem I reaction-centre complex showed the following characteristics. (1) The molecular mass required for electron transport from P-700 to iron-sulphur centre A was below 40 kDa. (2) Independent inactivation of iron-sulphur centres A and B was observed indicating their location on separate polypeptides. (3) The molecular mass of the polypeptides containing iron-sulphur centres A and B were 5–10 kDa based on a linear electron-transfer chain or 15–20 and 5–10 kDa (centre B) based on a branched chain. (4) A reaction centre ‘core’ containing the electron carriers for electron transport from P-700 to iron-sulphur centre X was indicated. These observations are discussed in comparison to current ideas on the polypeptide composition of the Photosystem I reaction centre. It is concluded that the radiation inactivation technique did not measure the size of Photosystem I polypeptides binding chlorophyll accounting for the small overall target size. The observed functional size came mostly from inactivation of the iron-sulphur centres showing that they are located on separate polypeptides.