Sequence of the precursor of the chloroplast thylakoid lumen protein plastocyanin

Abstract

In photosynthetic eukaryotes most chloroplast proteins are encoded by nuclear DNA^1,2. These proteins are synthesized as precursor proteins that are post-translationally taken up by chloroplasts and directed towards their functional compartment^2–4. One of these proteins, plastocyanin, is a photosynthetic electron carrier that is functional in the thylakoid lumen⁵ and therefore probably has to cross three membranes to arrive at its final location⁶. We have isolated a plastocyanin-specific complementary DNA clone from a Silene pratensis (white campion) cDNA library using a mixed oligonucleotide hybridization probe and hybrid-released translation. The isolated clone contained the nucleotide sequence for the mature protein and the complete transit peptide, which consists of a hydro phobic, 66-residue-long stretch of amino acids interspersed with positively charged residues. This precursor protein is most probably processed between two alanine residues. Comparison of the transit peptide of the plastocyanin precursor protein (in the thylakoid lumen) with those of the precursor proteins for ferredoxin and the small subunit of ribulose bisphosphate carboxylase (in the stroma) and with that of the chlorophyll a/b binding protein (inserted in the thylakoid membrane) revealed no similarities, except for a region of 14 residues at the aminoterminal end.