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Title: STRUCTURAL MODEL OF CYTOCHROME B559 BASED ON A MUTANT WITH GENETICALLY FUSED SUBUNITS

Author
item MACNAMARA, V - PLANT BIOLGOY UOFI URBANA
item SUTTERWALA, F - WASHINGTON U ST LOUIS MO
item PAKRASI, H - WASHINGTON U ST LOUIS MO
item WHITMARSH, CLIFFORD

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Crops can be damaged by excess light, leading to loss of photosynthetic capacity. In a previous study we described a molecular switch that controls the rate of photodamage. The switch is a heme protein, known as cytochrome b559, that is found in all plants. We proposed that the cytochrome acts as a safety valve, protecting against photodamage by deactivating a potentially damaging high energy state. Here we present a structural model cytochrome b559. To develop the model we created a mutant in which the two genes that code for the cytochrome are fused together. The fusion mutant was photosynthetically active, but was highly susceptible to damage by light. Based on spectroscopic measurements, genetic data, and computer modeling, we propose that each photosystem II reaction center contains two different cytochrome b559 proteins, oriented on opposite sides of the photosynthetic membrane. Knowledge of the structure of cytochrome b559 in photosystem II opens the door for developing genetically engineered plants designed to perform well in light that exceeds the photosynthetic capacity of the plant.

Technical Abstract: Photosystem II is a multi subunit protein complex located in the photosynthetic membrane of plants, algae and cyanobacteria. Using light energy, the photosystem II reaction center catalyzes the oxidation of water and the reduction of plastoquinone. A key component of photosystem II is cytochrome b559, a membrane bound heme protein required for assembly of the reaction center. The cytochrome is unusual in that its heme links two separate polypeptide subunits, either as a heterodimer or as two homodimers. To determine the structural organization of cytochrome b559, we used site-directed mutagenesis to fuse the coding regions of the two genes. Cells containing the fusion protein grow photoautotrophically and assemble functional photosystem II reaction centers. Optical spectroscopy shows that the fusion protein binds heme and is incorporated into photosystem II. Based on these observations we propose a structural model of cytochrome b559 in which one heme is coordinated to a homodimer made of two 9 kDa subunits, and a second heme is coordinated to a homodimer made of two 5 kDa subunits. In this model each photosystem II complex contains two cytochrome b559 hemes which are located on opposite sides of the photosynthetic membrane.