To gain insights into the molecular details of photosystem I (PSI) biogenesis, we characterized the PsbP-domain protein1 (ppd1) mutant of Arabidopsis thalianathat specifically lacks PSI activity. Deletion of PPD1 results in an inability of the mutant to grow photoautotrophically and a specific loss of the stable PSIcomplex. Unaltered transcription and translation of plastid-encoded PSI genes indicate that PPD1 acts at the posttranslational level. In vivo protein labeling experiments reveal that the rate of synthesis of PSI reaction center proteins PsaA/B in ppd1 is comparable to that of wild-type plants, whereas the rate of turnover of PsaA/B proteins is higher in ppd1 than in wild-type plants. With increasing leaf age, PPD1 content decreases considerably, while PSI content remains constant. PPD1 is a nuclear-encoded thylakoid lumenal protein and is associated with PSI but is not an integral subunit of PSI. Biochemical and molecular analyses reveal that PPD1 interacts directly and specifically with PsaB and PsaA. Yeast two-hybrid experiments show that PPD1 interacts with some lumenal loops of PsaB and PsaA. Our results suggest that PPD1 is a PSI assembly factor that assists the proper folding and integration of PsaB and PsaA into the thylakoid membrane.