PsbA

psbA is a gene encoding the Photosystem II protein D1 (also called PsbA) in the chloroplast genomes of plants and algae and in the genomes of cyanobacteria. It is one of the core components of the photosystem II reaction center and is critical for oxygenic photosynthesis.

The D1 protein forms part of the PSII core together with D2; it binds essential cofactors, participates in binding the Mn4CaO5 cluster, and mediates initial charge separation and electron transfer from water to plastoquinone. D1 is frequently damaged by light and has a rapid turnover, making the PSII repair cycle essential for maintaining photosynthetic efficiency.

Genomic organization and expression: In plants, psbA is encoded in the chloroplast genome and is transcribed by plastid RNA polymerase; translation occurs on chloroplast ribosomes and the product is inserted into the thylakoid membrane. In cyanobacteria, psbA exists in multiple gene copies (psbA1, psbA2, psbA3) with differential light responses and regulation.

Phylogeny and practical use: psbA sequences are used in phylogenetic analyses of photosynthetic organisms, and the related psbA-trnH spacer is employed as a plant DNA barcode region, aiding species identification and comparative studies.

Herbicide resistance: Mutations in psbA can alter the D1 protein such that binding of PSII inhibitors, including atrazine and DCMU, is reduced, conferring herbicide resistance in some plant lines and cyanobacteria.