Chloroplast-encoded small subunits of the three multiprotein complexes in photosynthetic electron transport

The photosystem I, photosystem II, and cytochrome b₆f complexes that are involved in electron transport of oxygenic photosynthesis consist of a number of subunits encoded by either the chloroplast or nuclear genomes. In addition to the major subunits that carry redox components or photosynthetic pigments, these complexes contain several to more than ten subunits with molecular masses of less than 10 kDa. Directed mutagenesis has served as a powerful tool for investigation of the roles of these small subunits in the organization or function of the complexes. Various chloroplast transformants of the green alga Chlamydomonas reinhardtii and mutants of cyanobacteria in which a gene encoding a small subunit was deleted or altered have been constructed. Evidence has accumulated suggesting that these small subunits function in the assembly, stabilization, or protection from photoinhibition of the complexes or in the modulation or regulation of electron transport. This article presents an overview of the properties and functions of the chloroplast encoded small subunits of the three multiprotein complexes of photosynthetic electron transport that have been mainly analyzed with chloroplast transformants of C. reinhardtii and the corresponding cyanobacterial transformants.