Characterization of a four-member psbA gene family from the cyanobacterium Anabaena PCC 7120.

The cyanobacterium Anabaena PCC 7120 contains a multigene family that encodes the D1 polypeptide of Photosystem II. This family consists of four members denoted psbAI-IV that are each unique but highly homologous. psbAII, III and IV are more closely related to each other than to psbAI. These three copies encode identical polypeptides that differ from the psbAI product by 21 amino acids. The transcription initiation site for psbAI has been mapped to 64-65 nucleotides upstream from the coding region. Primer extension assays performed with an oligonucleotide specific for psbAII, III and IV transcripts suggest that one or more of these genes is also expressed. Genomic mapping and chromosome walking experiments demonstrate that none of the four psbA copies is within 20 kbp of another member of the gene family.