Virescence, a phenotype in which leaves green more slowly than usual, is recognized to play a role inprotection from photo-oxidative damage before healthy chloroplasts are developed. The elucidation of themolecular mechanisms underlying virescence will provide insights into how the development of chloroplastsis controlled. In this study, we find that knockout alleles of Yellow Seedlings 1 (YS1) in Arabidopsis lead to avirescent phenotype, which disappears by 3 weeks after germination. The ys1 mutation resulted in markeddecreases in photosynthetic capacity and photosynthetic pigment complexes, and disturbed ultrastructure ofthylakoid membranes in 8-day-old seedlings. However, cotyledons of ys1 seedlings pre-treated in the dark for5 days turn green almost as fast as the wild type in light, revealing that the developmental defects in ys1 arelimited to the first few days after germination. Inspection of all known plastid RNA editing and splicing eventsrevealed that YS1 is absolutely required for editing of site 25992 in rpoB transcripts encoding the beta subunitof the plastid-encoded RNA polymerase (PEP). YS1 is a nuclear-encoded chloroplast-localized pentatricopeptiderepeat protein differing from previously described editing factors in that it has a C-terminal DYW motif.A defect in PEP activity is consistent with the changes in plastid transcript patterns observed in ys1 seedlings.We conclude that the activity of PEP containing RpoB translated from unedited transcripts is insufficientto support rapid chloroplast differentiation.