A chloroplast photosystem I reaction center mutation, ac-u-g-2.3, of Chlamydomonas reinhardtii has been complemented with a wild type psaB gene to restore photosynthetic competence. The mutation was mapped in the psaB coding sequence by chloroplast transformation using subcloned restriction fragments of psaB. The mutation was found to be a single base pair deletion resulting in a reading frame shift and premature termination of the polypeptide. Transformants were verified by insertion of a site-directed mutation which created a new restriction enzyme site. These transformations demonstrate the feasibility of insertion of site-directed mutations into the psaB gene in order to elucidate amino acid residues involved in photosystem I assembly and function.