IMPORT OF THE BARLEY PSI-F SUBUNIT INTO THE THYLAKOID LUMEN OF ISOLATED-CHLOROPLASTS

A full-length cDNA clone encoding the PSI-F subunit of barley photosys tem I has been isolated and sequenced. The open reading frame encodes a precursor polypeptide with a deduced molecular mass of 24 837 Da, Th e barley PSI-F precursor contains a bipartite presequence with charact eristics similar to the presequences of proteins destined to the thyla koid lumen. In vitro import studies demonstrate that an in vitro synth esized precursor is transported across the chloroplast envelope and di rected to the thylakoid membrane, where it accumulates in a protease-r esistant form. Incubation of the precursor with a chloroplast stromal extract results in processing to a form intermediate in size between t he precursor and mature forms. Hydrophobicity analysis of the barley P SI-F protein reveals a hydrophobic region predicted to be a membrane s panning alpha-helix, The hydrophobic nature of PSI-F combined with a b ipartite presequence is unusual. We postulate that the second domain i n the bipartite presequence of the PSI-F precursor proteins is require d to ensure the proper orientation of PSI-F in the thylakoid membrane. The expression of the PsaF gene is light-induced similar to other bar ley photosystem I genes.