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. Associates with the stromal face of the thylakoid membrane. Pale green phenotype.