The primary structure of a 4.0-kDa photosystem I polypeptide encoded by the chloroplast psaI gene

Partial amino acid sequences have been determined for a 4.0-kDa photosystem I polypeptide from barley. A comparison with the sequence of the chloroplast genome of Nicotiana tabacum and Marchantia polymorpha identified the polypeptide as chloroplast-encoded. We designate the corresponding gene psaI and the polypeptide PSI-I. The barley chloroplast psaI gene was sequenced. The gene encodes a polypeptide of 36 amino acid residues with a deduced molecular mass of 4008 Da. The 4.0-kDa polypeptide is N-terminally blocked with a formyl-methionine residue. Plasma desorption mass spectrometry established that the polypeptide is not post-translationally processed except for possible conversion of a methionine residue into methionine sulfone. The hydrophobic 4.0-kDa polypeptide is predicted to have one membrane-spanning alpha-helix and is homologous to transmembrane helix E of the D2 reaction center polypeptide of photosystem II.