Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Plant and Microbial Biosciences

Language

English (en)

Date of Award

January 2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Himadri Pakrasi

Abstract

Cyanobacteria, the only prokaryotes capable of oxygenic photosynthesis, are present in diverse ecological niches and play crucial roles in global carbon and nitrogen cycles. To proliferate in nature, cyanobacteria utilize a host of stress responses to maintain photosynthesis under periodic changes in environmental conditions. Recent advances in proteomic study have enabled a systems-level analysis of cellular functions in many systems. Because proteins are directly responsible for cellular functions, measurements of protein abundances provide significant clues to the modulation of cellular functions during different environmental perturbations. A detailed knowledge of the composition of, as well as the dynamic changes in, the proteome is necessary to gain fundamental insights into such stress responses. Toward this goal, we have performed a large-scale proteomic analysis of the widely studied model cyanobacterium Synechocystis sp. PCC 6803 under 33 different environmental conditions. Photosystem II: PSII) is a large membrane protein complex that performs the water oxidation reactions of the photosynthetic electron transport chain in cyanobacteria, algae, and plants. Subsequently, we also performed an accurate mass tag: AMT) high-sensitivity proteomic analysis of PSII complexes purified from the cyanobacterium Synechocystis sp. PCC 6803. Taken together, these proteomics studies revealed novel information into the function and assembly of Photosystem II. We identified six PSII associated proteins that are encoded by a single operon containing nine genes, slr0144 to slr0152. This operon encodes proteins that are not essential components of the PSII holocomplex but accumulate to high levels in precomplexes lacking any of the lumenal proteins PsbP, PsbQ, or PsbV. Genetic deletion of this operon shows that removal of these protein products does not alter photoautotrophic growth or PSII fluorescence properties. Nonetheless these proteins confer fitness under competition in high light intensities. However, the deletion mutation does result in decreased PSII-mediated oxygen evolution and an altered distribution of the S states of the catalytic Mn cluster. PSII complexes isolated from Δslr0144 - slr0152 also show decreased photosynthetic capacity and altered polypeptide composition. These data demonstrate that the proteins encoded by the genes in this operon are necessary for optimal function of PSII and function as accessory proteins during assembly of the PSII complex. Based on these results, we have named the products of the slr0144 - slr0152 operon Pap: photosystem II assembly proteins). Additionally, through this proteomics study, we identified the protein sll1390, which we have named Psb32. To investigate its function, we analyzed subcellular localization of Psb32 and the impact of genetic deletion of the psb32 gene on PSII. Psb32 is an integral membrane protein, primarily located in the thylakoid membranes. Although not required for cell viability, Psb32 protects cells from oxidative stress and additionally confers a selective fitness advantage in mixed culture experiments. Specifically, Psb32 protects PSII from photodamage and accelerates its repair. Thus, we propose that Psb32 plays an important role in minimizing the effect of photoinhibition on PSII. Together, the proteins of the pap operon and Psb32 represent new components in PSII assembly and function.

Comments

Permanent URL: http://dx.doi.org/10.7936/K7BC3WJ6

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