Functions as an electron carrier between membrane-bound cytochrome b6-f and photosystem I in oxygenic photosynthesis. Thought to function as a monomer, however 2 crystal forms are observed; a homodimer and homotrimer, suggesting the protein oligomerizes. Induced in copper-deficient medium. Binds 1 heme c group covalently per subunit. Belongs to the cytochrome c family. PetJ subfamily.