Nucleotide sequence of thepsbH gene of the cyanobacteriumSynechocystis6803

In green photosynthetic eukaryotes the psbH gene encodes the so-called '9 kDa phosphoprotein' of photosystem two, a polypeptide which undergoes reversible light-regulated phosphorylation (1). The gene shows a conserved location in the co-transcribed chloroplast DNA psbB-psbH-petB-petD operon. A 4.2 Kb Hind HI fragment was isolated from a Synechocystis 6803 XEMBL3 genomic library using the Triticum aestivum psbH gene as a probe. Sequence analysis of a portion of this fragment revealed the psbH gene to consist of 64 codons, preceded by a Shine-Dalgarno like sequence (SD). Concurrent genomic Southern blots indicated this gene is single-copy. Assuming the N-terminal methionine is post-translationally removed, the polypeptide has a predicted MW of 6895 Da. It has 72% amino acid identity (67% DNA homology) over residues Arg-3 to Gly-56 with the corresponding T. aestivum polypeptide region, diverging and extending 3 amino acids further at the Cterminus. Its similar hydropathy profile also suggests one transmembrane helix. Significantly the polypeptide appears truncated at the N-terminus and is missing the phosphorylation site at Thr-2 seen conserved in the plant and Chlamydomonas reinhardtii sequences (2, 3). This explains why the gene product was not detected in Western blots of Synechocystis 6803 thylakoid membranes using antibodies from Dr J. Bennett raised against a synthetic peptide corresponding to the first ten residues of the spinach psbH protein (Dr A. Telfer personal communication). Eighteen of the first twenty amino acids are identical to the Synechococcus vulcanus sequence (4). With this N-terminal data for a second species it seems unlikely that the psbH gene product is phosphorylated in cyanobacteria. In addition the psbB gene is not located within 11 kb upstream or downstream of the psbH gene in the Synechocystis 6803 genome. The presence of the psbH gene in cyanobacteria argues for the product having a similar function in photosystem two of all oxygenie photosynthetic organisms. Speculations on function which are not applicable for cyanobacteria, such as a specific involvement in energy transfer from LHC II to the photosystem two reaction centre (5), therefore become less tenable.