Structure and biogenesis of Chlamydomonas reinhardtii photosystem I

The photosystem I complex of the green alga Chlamydomonas reinhardtii was isolated and fractionated into its two subcomplex components: the core complex (CC I), which contained the reaction center (P‐700) and had four polypeptide subunits, and the light‐harvesting complex (LHC I) which contained four polypeptides of about 22, 25, 26 and 27 kDa. The 22‐kDa apoprotein was isolated as a chlorophyll a and b binding protein. In the isolated photosystem I holocomplex, about ten copies of the 22‐kDa LHC I apoprotein are present for each CC I unit. The 22‐kDa polypeptide as well as the other three polypeptides of this complex and the subunit II of CC I are translated on 80S cytoplasmic ribosomes, and therefore are coded in the nucleus. During the greening process of the Chlamydomonas reinhardtii y‐1 mutant the 22‐kDa LHC I polypeptide, which cross‐reacts with polyclonal antibodies raised against the Lemna gibba 20‐kDa LHC I apoprotein, accumulates in thylakoids at a late stage of their development, and about 2–3 h after the LHC II and CC I subunit II polypeptides have accumulated. Accumulation of the 22‐kDa protein during greening is inhibited by cycloheximide but not by chloramphenicol.