Nucleotide sequence of the psbA3 gene from the cyanobacterium Synechocystis PCC 6803.

The psbA gene which encodes the Dl polypeptide of the photosystem two reaction center is present as three different copies in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (1). The nucleotide sequences for two of these copies, psbAl (2) and psbA2 (3), have been published. Here we present the nucleotide and deduced amino acid sequences for the remaining copy, psbA3, which is located on a 9.4 kb genomic MndlH fragment (1,4). The sequence between nucleotides 101 and 1249 was determined from plasmids isolated previously (4). The rest of the sequence was obtained from plasmids containing DNA that had been amplified using the inverted polymerase chain reaction (5). The psbA3 gene is highly homologous to the psbA2 gene (99.4% identity at the nucleotide level) and encodes an identical gene product. Thus Synechocystis 6803 contains psbA genes that code for two distinct forms of the Dl polypeptide. However, under normal laboratory growth conditions only the psbA2 and psbA3 genes appear to be expressed (6). 152 bp downstream of the stop codon for psb<A3 is the putative initiation codon (underlined) for an open reading frame (ORF)