Characterization of the Sequential Light-Regulated Assembly of Photosystem I Core Complex

Abstract

The biogenesis and assembly of the photosynthetic complexes of higher plants proceed by a multi-step process in the two intercellular compartments; the chloroplast and the cytoplasm. Hence a high degree of coordination is required between these different compartments in each step of the process. The process of biogenesis initiates with the transcription and translation of the various polypeptide subunits taking place both in the chloroplast (for the chloroplast-encoded subunits) and in the nucleus and cytoplasm (for the nuclear-encoded subunits) (Archer and Keegstra, 1991; Chitnis and Thornber, 1988). The polypeptide subunits synthesized in the cytoplasm are made as precursors having a leader (transit) sequence in their amino-terminus (Von-Heijne et al., 1989). These precursors are post-translationally imported into the chloroplasts in an energy dependent process (requiring ATP), probably via a receptor situated in the envelope membrane (Archer and Keegstra, 1991; Chitnis and Thornber, 1988). In the chloroplast the precursors enter the thylakoid membrane. They are processed into their mature form and associate with their cofactors (pigments and metal clusters) and with the chloroplast-encoded subunits to form the fully active complex. The order and mechanism in which these events occur, are not yet fully understood. Many experimental systems have been developed in order to try to dissect and follow the different stages of these complex processes and thereby determine the temporal sequence of the assembly of the complexes found in the membrane.