Nucleotide sequence of the psbH gene of the cyanobacterium Synechocystis 6803

Nucleotide sequence of the psbK gene of the cyanobacterium Synechocystis 6803 The psbK gene is found in chloroplast, cyanelle and cyanobacterial genomes (1-3). It encodes a low molecular weight photosystem H-associated polypeptide (1). This gene is closely linked to the psbl gene in chloroplast genomes. We have previously reported the isolation and restriction mapping of a Synechocystis 6803 AEMBL3 genomic library clone, XZZK1, which has homology to a Marchantia polymorpha psbK DNA probe (4). The hybridisation signal was localised to a 3.4 kb HindUl fragment. Nucleotide sequencing of part of this fragment revealed that the Synechocystis 6803 psbK gene consists of 45 codons, the same length as the Anacystis nidulans 6301 and Cyanophora paradoxa genes (5, 2). An upstream stop codon and a Shine-Dalgarno-like sequence (SD) make the assignment of the initiation codon unambiguous. In the immediate 3' flanking region there is a palindrome (P) and a short inverted repeat (IR) which overlap. From comparisons with the N-terminal psbK protein sequences of spinach, Triticum aestivum and Synechococcus vulcanus (6, 3) the mature polypeptide is likely composed of 37 amino acids with cleavage of the pre-sequence occurring after residue Ala-8 (cleavage site arrowed). Hydropathy plots suggest the mature protein has one membrane-spanning domain. The predicted mature sequences are well-conserved across species, with that of Synechocystis 6803 having 92% identify with A. nidulans 6301 (5); 78% with C. paradoxa (2); 76% with M. polymorpha (7) and with Sinapis alba (8); 70% with Nicotiana tabacum (9) and with Oryza sativa (10). All have the same number of residues. The pre-sequence length of eight amino acids is the same as those predicted for A. nidulans and C. paradoxa. This is appreciably shorter than chloroplast psbK pre-sequences which may, it has been speculated, serve to position the amino terminus on the lumenal side of the thylakoid membrane (3). Hybridisation experiments and additional DNA sequencing of the psbK flanking sequences suggest that the psbl gene is not closely linked to psbK in Synechocystis 6803. ACKNOWLEDGEMENTS We thank Professor K.Ohyama for providing the M. polymorpha psbK and psbl genes, Dr J.G.K.Williams for the Synechocystis 6803 XEMBL3 library and Dr D.A.Bryant for the C. paradoxa data before publication.