Identification of a Small Tetraheme Cytochromec and a Flavocytochrome c as Two of the Principal Soluble Cytochromes c inShewanella oneidensis Strain MR1

ABSTRACT Two abundant, low-redox-potential cytochromesc were purified from the facultative anaerobeShewanella oneidensis strain MR1 grown anaerobically with fumarate. The small cytochrome was completely sequenced, and the genes coding for both proteins were cloned and sequenced. The small cytochrome c contains 91 residues and four heme binding sites. It is most similar to the cytochromes c fromShewanella frigidimarina (formerly Shewanella putrefaciens) NCIMB400 and the unclassified bacterial strain H1R (64 and 55% identity, respectively). The amount of the small tetraheme cytochrome is regulated by anaerobiosis, but not by fumarate. The larger of the two low-potential cytochromes contains tetraheme and flavin domains and is regulated by anaerobiosis and by fumarate and thus most nearly corresponds to the flavocytochromec-fumarate reductase previously characterized fromS. frigidimarina to which it is 59% identical. However, the genetic context of the cytochrome genes is not the same for the twoShewanella species, and they are not located in multicistronic operons. The small cytochrome c and the cytochrome domain of the flavocytochrome c are also homologous, showing 34% identity. Structural comparison shows that theShewanella tetraheme cytochromes are not related to theDesulfovibrio cytochromes c 3but define a new folding motif for small multiheme cytochromesc.

Crystallization and preliminary X-ray crystallographic analysis of a periplasmic tetrahaem flavocytochrome c 3 from Shewanella frigidimarina NCIMB400 which has fumarate reductase activity

The fumarate reductase of Escherichia coli and other bacteria is a membrane-bound enzyme consisting of four subunits. A soluble periplasmic 64 kDa tetrahaem flavocytochrome c 3 from Shewanella frigidimarina NCIMB400 which possesses a catalytic fumarate reductase activity has been crystallized. The crystals belong to space group P212121 with unit-cell parameters a = 72.4, b = 110.1, c = 230.2 Å. Assuming a molecular dimer in the asymmetric unit, the crystals contain 65% solvent and, when cryocooled to 100 K, the crystals diffract to at least 3.0 Å resolution. The crystals, however, display an inherent lack of isomorphism and the plausibility of a MAD phasing experiment has therefore been investigated by measuring the iron K absorption edge from a single crystal.