The PSI-K subunit of photosystem I from barley (Hordeum vulgare L.). Evidence for a gene duplication of an ancestral PSI-G/K gene.

Photosystem I of barley contains a polypeptide with an apparent molecular mass of 7 kDa when isolated using the detergent n-decyl-beta-D-maltopyranoside. The 7-kDa polypeptide is lost from the PS I complex isolated using Triton X-100. The 7-kDa polypeptide and a corresponding full-length cDNA clone have been isolated. Based on high sequence similarity to an N-terminal sequence of PSI-K from spinach and to the deduced amino acid sequence of Psak from Chlamydomonas reinhardtii the 7-kDa barley polypeptide is identified as PSI-K. The cDNA clone encodes a precursor polypeptide of 131 amino acid residues with a calculated molecular mass of 13,726 Da. The transit peptide shows characteristics of polypeptides imported into the chloroplast. PSI-K has two hydrophobic regions predicted to be membrane-spanning alpha-helices. In vitro expressed prePSI-K polypeptide was imported into intact chloroplasts, whereas an in vitro expressed prePSI-K lacking 7 amino acid residues (Met-Ala-Ser-Gln-Leu-Ser-Ala) at the N-terminal end of the transit peptide failed to be imported. The mRNA encoding PSI-K increases during illumination. PsaK is located in a single locus in the genome. PSI-K has significant similarity to PSI-G. When comparing the barley PSI-K and PSI-G with the reported PSI-K sequence from Synechococcus vulcanus, the degree of similarity is equal, suggesting that an ancestral gene has been duplicated in a chloroplast progenitor but not in a cyanobacterial.