Essential for the assembly of the photosystem I (PSI) complex. May act as a chaperone-like factor to guide the assembly of the PSI subunits. Interacts with Y3IP1. Non-photosynthetic phenotype due to the absence of photosystem I (PSI). Can grow only in vitro on sterile sucrose-containing medium. Under normal light conditions, mutant plants show severe pigment deficiency and leaf bleaching due to the accumulation of photooxidative damage. Under low light conditions, mutant plants show a light-green phenotype, very slow growth and delayed development. Belongs to the Ycf3 family.