This domain is a heme binding cytochrome known as cytochrome c550, or cytochrome c549, or PsbV [1].

1: Crystal structure of low-potential cytochrome c549 from Synechocystis sp. PCC 6803 at 1.21 A resolution. Frazao C, Enguita FJ, Coelho R, Sheldrick GM, Navarro JA, Hervas M, De la Rosa MA, Carrondo MA; J Biol Inorg Chem. 2001;6:324-332. PMID:11315568