The Photosystem I 5.5 kDa subunit (the psaK gene product). An intrinsic subunit of the PSI reaction center complex

Previous studies of the Photosystem I reaction center complex, the chlorophyll protein 1 complex (CPI), have shown it is a heterodimer of two high-molecular-mass subunits of 83000 and 82000, the psaA and psaB gene products. The present work has identified another intrinsic low-molecular-mass subunit of 5 500 (as determined by SDS-PAGE) that is tightly associated with the CPI complex. This subunit can be separated from CPI after treatment with thiol agents. CPI complexes prepared either in the presence or absence of thiols show no differences in photochemical activity under steady-state actinic illumination. A comparison of the N-terminal amino acid sequence of the 5.5 kDa subunit with the sequence of a Chlamydomonas PSI subunit, P37, identifies this low-molecular-mass subunit as the PSI psaK gene product.