Nuclear genome-encoded factor required for the accumulation of photosystem I (PSI). Functions as PSI biogenesis factor (PubMed:16679416, PubMed:28522455). Cooperates with PSA3 to promote the stable assembly of PSI in the thylakoid membrane. May target primarily the PsaC subunit (PubMed:28522455). Interacts with PSA3. Copurifies with PSI. Induced by light. Seedling lethality when homozygous. Dwarf and pale yellowish phenotype when grown on MS medium supplemented with 2% sucrose.