i. Rubredoxin was isolated from the facultative anaerobe Spirochaeta aurantia and from the obligate anaerobe Spi~vchaeta stenostrepta. The spirochetal rubredoxins contained one atom of iron per molecule and no inorganic sulfide. Their molecular weight (approximately 6000), spectral properties, and amino acid composition resembled those of rubredoxins obtained from anaerobic bacteria. A rubredoxin or & rubredoxin-like protein was present in the host-associated Treponema denticola.
Rubredoxin was detected in extracts of both anaerobically-and aerobically-grown cells of N. aurantia.
- An unstable ferredoxin was isolated from cell extracts of anaerobically-grown S. aurantia, but was not detected in aerobically-grown cells. The spectral characteristics and amino acid composition of this protein were similar to those of other bacterial ferredoxins. Four atoms of iron and four acid-labile sulfide residues were present per molecule. The molecular weight of S. aurantia ferredoxin was near 6000. S. aurantia ferredoxin preparations stimulated the formation of acetyl phosphate from pyruvate by diethylaminoethyl cellulose-treated extracts of Ulostridium butyricum, whereas purified spirochetal rubredoxin did not.
The non-heme iron proteins ferredoxin and rubredoxin participate in the metabolic processes of numerous anaerobic bacteria. Ferredoxin serves as electron carrier in a variety of oxidation-reduction reactions (Buchanan and Arnon, 1970). Rubredoxin functions in electron transfer (Lovenberg and Sobel, 1965), but no specific physiological role is known for this protein in anaerobic bacteria.
Rubredoxins have been found in anaerobic bacteria as diverse as Chlorobium thiosul/atophilum, Chloropseudomonas ethylicum, Clostridium sticklandii, Clostridium pasteurianum, Desul/ovibrio ffigas, Desul/ovibrio desul/uricans, Micrococeus lactilyticus, Micrococcus aerogenes, and Peptostreptococcus elsdenii (Meyer et al., 1971, and references therein). All the above-mentioned organisms contain ferredoxin, in addition to rubredoxin (Meyer et al., 1971 ). Recently, in the course of investigations on the metabolism of free-living spirochetes, rubredoxins, but not ferredoxins, were