Required for the biogenesis and accumulation of native cytochrome b6 in the thylakoid membrane. Controls the conversion of apocytochrome b6 to holocytochrome b6. Required for covalent binding of the c-type heme to cytochrome b6. High chlorophyll fluorescence phenotype due to an impaired photosynthetic electron flow (PubMed:17971335). Seedling lethal when grown on soil. On agar plates supplied with sucrose, seedlings grow very slowly with a chlorotic phenotype. Deficiency in the accumulation of the subunits of the cytochrome b6f complex and lack of covalent heme binding to cytochrome b6 (PubMed:17971335, PubMed:18593701).