The plastid-encodedpsaJgene encodes a hydrophobic low-molecular-mass subunit of photosystem I (PSI) containing one transmembrane helix. Ho-moplastomic transformants with an inactivatedpsaJgene were devoid of PSI-J protein. The mutant plants were slightly smaller and paler than wild-type because of a 13% reduction in chlorophyll content per leaf area caused by an 20% reduction in PSI.