Molecular characterization of the nuclear-encoded photosystem II subunit PSII-W in Chlamydomonas reinhardtii

Abstract

Photosystem II catalyses the light-driven electron transfer from water to plastoquinone. The complex contains at least 25 different polypeptides; the function of many of these has yet to be established. In eukaryotes the components of PSII are encoded for both chloroplast and nuclear genes. PSII-W is a nuclear-encoded subunit that is thought to be located near to the core of the photosystem II complex. However a recent paper has suggested this subunit is a component of photosystem I (Hiyama et al., 2000. Biochim. Biophys. Acta. 1459(1): 117-24). We have screened a cosmid library, then cloned and sequenced the genomic and coding sequences for the psbW gene in the photosynthetic alga Chlamydomonas reinhardtii. Analysis has established that the psbW gene is present as a single copy gene in the nuclear genome. Using the derived protein sequence, which predicts that the mature protein is 6.1kDa, topological analysis determined the putative cleavage sites within the targeting sequence, and indicates that the mature protein is present as a single transmembrane protein. We have over-expressed the PSII-W protein in Escherichia coli and using an antibody raised against this protein, we establish that PSII-W is located exclusively in the PSII complex of C. reinhardtii.