ycf1 is a chloroplast-encoded large open reading frame. Recently, the Ycf1 protein was identified as a component (Tic214) of the translocon complex in the inner envelope membrane of chloroplasts (TIC). Here, by construction of transplastomic ycf1 overexpression (OE) lines in tobacco, we show that the protein level of Ycf1 is tightly regulated. Slight overaccumulation of the Ycf1 protein leads to strongly inhibited translation of chloroplast-encoded core subunits of the photosynthetic complexes, thus causing a massive repression of photosystem biogenesis and photosynthetic capacity. The nucleus-encoded subunits of the photosystems, especially the light-harvesting complex (LHC) proteins, are much less affected and promote the formation of hyperstacked grana thylakoids in transplastomic ycf1 OE lines. Our data indicate that Ycf1 participates in the control of photosystem biogenesis by directly modulating the translation of the core subunits of photosynthetic complexes in the chloroplast, or alternatively, by affecting the client selectivity of the protein import machinery.