Main chain fold of a [2Fe-2S]ferredoxin I from Aphanothece sacrum at 2.5 A resolution.

Crystal structure analysis of a [2Fe-2S]ferredoxin I from Aphanothece sacrum, a blue green alga, was carried out at 2.5 A resolution. The phase angle of each reflection was determined by the single isomorphous replacement method coupled with the anomalous dispersion effect for the uranium derivative. The four molecules in an asymmetric unit were clearly seen in a 3.9 A electron density map. The main chains of three molecules were traced at 2.5 A resolution. The structure of the remaining one molecule, however, remains unknown because of the poor electron density of the corresponding region. The three main chain folds exhibit the same topology as that in Spirulina platensis. The structural similarity between A. sacrum and S. platensis ferredoxins, whose amino acid sequences are different from each other by about 30%, strongly suggests that all plant-type ferredoxins have the same main chain fold.