We have isolated a minor soluble green-colored heme protein (GHP) from the purple sulfur bacterium, Halochromatium salexigens, which contains a c-type heme. A similar protein has also been observed in the purple bacteria Allochromatium vinosum and Rhodopseudomonas cryptolactis. This protein has wavelength maxima at 355, 420, and 540 nm and remains unchanged upon addition of sodium dithionite or potassium ferricyanide, indicating either an unusually low or high redox potential, respectively. The amino-acid sequence indicates one heme per peptide chain of 72 residues and reveals weak similarity to the class I cytochromes. The usual sixth heme ligand methionine in these proteins appears to be replaced by a cysteine in GHP. Only one known cytochrome has a cysteine sixth ligand, SoxA (cytochrome c-551) from thiosulfate-oxidizing bacteria, which is low-spin and has a high redox potential because of an un-ionized ligand. The native size of GHP is 34 kDa, its subunit size is 11 kDa, and the net charge is -12, accounting for its very acidic nature. A database search of complete genome sequences reveals six homologs, all hypothetical proteins, from Oceanospirillum sp., Magnetococcus sp., Thiobacillus denitrificans, Dechloromonas aromatica, Thiomicrospira crunogena and Methylobium petroleophilum, with sequence identities of 35-64%. The genetic context is different for each species, although the gene for GHP is transcriptionally linked to several other genes in three out of the six species. These genes, coding for an RNAse, a protease/chaperone, a GTPase, and pterin-4a-carbinolamine dehydratase, appear to be functionally related to stress response and are linked in at least 10 species.