Membrane-Associated c-type Cytochromes from the Green Sulfur Bacterium Chlorobium limicola forma thiosulfatophilum: Purification and Characterization of Cytochrome c553

Tetraheme cytochromes involved in photosynthetic electron transport have previously been described associated with the reaction centers of purple photosynthetic bacteria; however, similar heme proteins have not until now been characterized in the phylogenetically distinct green sulfur bacteria. In this paper we describe the first isolation and characterization of a multitheme, membrane-associated cytochrome from a green sulfur bacterium, Chlorobium limicola forma thiosulfatophilum. We show that this cytochrome contains a single polypeptide of 32 kDa apparent molecular mass on SDS-PAGE and has a characteristic broad alpha-band absorption at 553 nm. By both low-temperature absorption and electron paramagnetic resonance spectroscopy, we demonstrate that there are at least four distinct heme groups.