A new plant-type ferredoxin from halobacteria.

A stable, 2Fe-type ferredoxin has been prepared from Halobacterium halobium and purified by chromatography. A similar ferredoxin was also found in three other Halobacteria. The ferredoxin is present in large amounts-about 1 percent of the total soluble protein. From amino acid composition a molecular weight of 14800 +/- 200 was calculated. The ferredoxin was found to contain two atoms each of iron and sulphide. The midpoint redox potential of the protein is about -345 mV. The electron paramagnetic resonance spectrum of the reduced form shows much similarity to plant and algal ferredoxins with gx = 1.90, gy = 1.97 and gz = 2.07. The same similarity is observed in the optical absorption, optical rotatory dispersion and circular dichroism spectra. However it does not seem to mediate electron transport in the NADP-photoreduction system of chloroplasts. Extracts of the bacterial cells catalyze the reduction of the ferredoxin by NADH.