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 molecu lar mass of 7 kDa when isolated using the detergent n-decyl-beta-D-mal topyranoside. The 7-kDa polypeptide is lost from the PS I complex isol ated using Triton X-100. The 7-kDa polypeptide and a corresponding ful l-length cDNA clone have been isolated. Based on high sequence similar ity 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 pr ecursor polypeptide of 131 amino acid residues with a calculated molec ular mass of 13,726 Da. The transit peptide shows characteristics of p olypeptides imported into the chloroplast. PSI-K has two hydrophobic r egions predicted to be membrane-spanning alpha-helices. In vitro expre ssed prePSI-K polypeptide was imported into intact chloroplasts, where as an in vitro expressed prePSI-K lacking 7 amino acid residues (Met-A la-Ser-Gln-Leu-Ser-Ala) at the N-terminal end of the transit peptide f ailed to be imported. The mRNA encoding PSI-K increases during illumin ation. PsaK is located in a single locus in the genome. PSI-K has sign ificant similarity to PSI-G. When comparing the barley PSI-K and PSI-G with the reported PSI-K sequence from Synechococcus vulcanus, the deg ree of similarity is equal, suggesting that an ancestral gene has been duplicated in a chloroplast progenitor but not in a cyano-bacterial.