Functions as an electron carrier between membrane-bound cytochrome b6-f and photosystem I in oxygenic photosynthesis. E(0) is +314 mV. Monomer. Homodimer, or even higher oligomerization, in crystal structures (PubMed:11478889, PubMed:12077429). Binds 1 heme c group covalently per subunit. Belongs to the cytochrome c family. PetJ subfamily.