PSII-T, a new nuclear encoded lumenal protein from photosystem II. Targeting and processing in isolated chloroplasts.

An intronless nuclear gene, psbT, isolated from cotton, encodes a putative 11-kDa protein (PSII-T) highly homologous in its C terminus to the N terminus of the partially sequenced PSII-T protein from spinach photosystem II. Analysis of the deduced amino acid sequence of cotton PSII-T revealed the presence of potential chloroplast stroma and thylakoid targeting domains and a putative mature PSII protein of 3.0 kDa, composed of only 28 amino acid residues. The cotton PSII-T 11-kDa precursor was synthesized in vitro in a wheat germ extract translation system, and the translation product was used in assays for protein imports into isolated pea chloroplasts. It was shown that the cotton PSII-T precursor was imported into the chloroplasts, processed to a mature form of approximately 3.0 kDa, agreeing with the predicted size from amino acid sequence analysis, and localized to the lumenal side of the thylakoid membrane, thus defining a new nuclear encoded lumenal protein and the smallest polypeptide of PSII reported to date. Processing of the PSII-T precursor occurred in two steps and involved the formation of a stromal intermediate of approximately 7.5 kDa, as predicted from primary structure analysis.