Nuclear genome-encoded factor required for the accumulation of photosystem I (PSI). Functions as PSI biogenesis factor. Cooperates with PYG7 to promote the stable assembly of PSI in the thylakoid membrane. May target primarily the PsaC subunit. Does not seem to be required for the expression of chloroplast genes encoding PSI subunits. Interacts with PYG7. Associates with the stromal face of the thylakoid membrane. Seedling lethality when homozygous. Pale green and slow growing phenotype when grown on MS medium supplemented with 2% sucrose.