Amino acid sequence of spinach ferredoxin:thioredoxin reductase variable subunit

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Abstract
Ferredoxin:thioredoxin reductase (FTR) is an iron‐sulfur protein, which, in the presence of ferredoxin and thioredoxin, catalyses the light‐dependent activation of several photosynthetic enzymes. Spinach FTR consists of two dissimilar polypeptide chains, A and B, present in equal amounts. Whereas subunit B seems to be responsible for the catalytic activity, subunit A has no known catalytic function. We found earlier that the N‐terminus of subunit A, also called the variable subunit, shows terminal redundancy and that 2–3 of its serine residues are phosphorylated [Tsugita, A. Yano, K., Gardet‐Salvi, L. & Schürmann, P. (1991) Protein Sequence Data Anal. 4, 9–13]. We now report the complete amino acid sequence of subunit A, determined by conventional protein sequencing methods. The polypeptide chain with a calculated molecular mass of 12 669 Da consists of 112 amino acids and has a calculated isoelectric point of 5.4. The analysis of the sequence supports the idea that this subunit has no catalytic function. The comparison with a known cyano‐bacterial FTR reveals about 58% similarity and the striking presence of a N‐terminal extension in the spinach protein. This extension may be responsible for the reported size variability of this subunit.