Amino acid sequence of the major component of Nostoc muscorum ferredoxin.

The amino acid sequence of the major component of ferredoxin isolated from a blue-green alga, Nostoc muscorum, grown under N2 as the sole nitrogen source has been studied. The use of a combination of sequence analyzer, carboxypeptidases, and manual Edman degradations on tryptic and chymotryptic peptides of carboxymethylferredoxin has established the amino acid seuqence, which consists of 98 amino acid residues. Only four cysteine residues were present, located at positions 41, 46, 49, and 79. These residues must fulfil the minimum requirement in this ferredoxin for the chelation of two iron atoms, as postulated previously. The sequence is similar to those of Spirulina ferredoxins in having two extra residues at positions 10 and 14 compared with other chloroplast-type ferredoxins. Sequence comparison among blue-green algal ferredoxins suggests that Nostoc muscorum ferredoxin is more closely related to Spirulina ferredoxins than to Aphanothece major ferredoxin.