The green unicellular alga Chlamydomonas reinhardtii offers interesting possibilities for studying the function, synthesis and assembly of the photosynthetic apparatus. Chloroplast and nuclear genes coding for subunits of photosystem II, a major multimolecular membrane-associated photosynthetic complex, have been isolated and characterized. Examination of these genes and of their expression in wild-type cells and in chloroplast and nuclear photosystem II mutants has revealed a complex regulatory circuitry between chloroplast and nucleocytoplasmic compartments. One of the photosystem II subunits which is involved in herbicide binding has been found to have distinct single amino acid substitutions in several herbicide resistant mutants. Some of these mutations do not appreciably affect photosynthetic yield and may therefore be of agronomic interest.