Amino Acid Sequence of Aphanothece sacrum Ferredoxin II (Minor Component)

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Abstract
The amino acid sequence of a ferredoxin (ferredoxin II, previously called minor component) from Aphanothece sacrum, a blue-green alga, was established by a combination of various conventional methods. The total number of residues was 99, giving a molecular weight of 10, 356, which is the largest reported for a chloroplast-type ferredoxin except for a halo-bacterial ferredoxin. This is the first time that the sequences of two ferredoxins isolated from one species of blue-green algae have been established. The ferredoxin contained one tryptophan and two methionine residues. The position of tryptophan in ferredoxin II was the same as in higher plant ferredoxins, but the methionines were in different positions from those in horsetail and Scenedesmus ferredoxins. No total charge difference was observed between ferredoxins I and II, though they were separated on a DEAE-cellulose column. In an alignment of the sequences of chloroplast-type ferredoxins, Aphanothece ferredoxin II had two extra residues at positions 10 and 14; this feature is common in ferredoxins of other blue-green algae and a red alga, but ferredoxin I had no extra residues and showed features in common with ferredoxins of a green alga and vascular plants. The two ferredoxins are probably genetically independent isozymes and two phylogenetic models are postulated in terms of the molecular evolution of Aphanothece ferredoxins with special reference to the existence of extra residues in various ferredoxins.