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. 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.