An alternative form of a 7Fe dicluster-type ferredoxin (Fd-B) with a different charge density was purified as a minor component from the aerobic and thermoacidophilic archaeon, Sulfolobus sp. strain 7. Comparison of its properties with those of native 7Fe ferredoxin (Fd-A), a major ferredoxin, was made in terms of the molecular properties, the absorption, circular dichroism and electron paramagnetic resonance spectral properties, and the reactivity coupled with the cognate 2-oxoacid:ferredoxin oxidoreductase at 50 degrees C. Our results suggest that the Sulfolobus 7Fe ferredoxin has two spectroscopically distinct forms with slightly different conformations. (C) 1995 Academic Press, Inc.