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Nucleotide sequencing of the second S~~ec!zocystis 6803 psbG gene, p&G2 shows the predicted polypeptide to be 219 amino acids long. It is less similar to chloroplast psbG genes than is the Synechocysllr: psbG1 copy. Alignment with seven other p&G protein sequences, including that from the Paramecium mitochondrial genome reveals a central highly conserved region common to each. This is discussed as evidence supporting the proposal that the psbG polypeptide is a NAD(P)H dehydrogenase (complex I) subunit in cyanobacteria, chloroplasts and mitochondria.Cyanobacterium; Gene, psbG; NADH dehydrogenase; Photosynthesis; Respiration; (~y~c~ocy~f~ 6803) I~RODUCT~ONThe p&G gene shows a conserved location between the n&C (or n&3) and an open reading frame of around 158 residues (ORFl58 in Nicotiana tabacum) in all the chloropl~t genomes in which it has been studied [l-5] Published by Elsevier Science Publishers 3. K (Biomedical Division) these assignments in our preliminary observations when we were unaware of parallel work.)ApsbG-like open reading frame in the mitochondrial genome of the ciliate Paramecium has been sequenced [lo]. We have aligned the predicted Paramecium p&G protein sequence with all the photos~thetic species' psbG sequences currently available, including Synechocystis psbG2. The high degree of homology across the central region of all the psbG sequences strongly suggests that the Paramecium gene encodes a functional product and supports the proposal that the p&G protein is a NAD(P)H dehydrogenase (complex I) subunit in cyanobacteria, chloroplasts and mitochondria.

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