Date of Award

12-8-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Qingfang He

Abstract

Cyanobacteria have evolved various acclimation mechanisms to enable them to thrive under harsh environmental conditions. There are a total of more than 80 sensory kinases and response regulators on the genome of the cyanobacterium Synechocystis sp. PCC 6803, which coordinate the molecular responses to changes in environmental conditions. However, it is not clearly understood how these regulators control the stress-responsive genes. This dissertation focuses on analysis of the functions of two regulators, RpaA and DspA, by targeted mutagenesis. Upon deletion of RpaA, a significant reduction in chlorophyll fluorescence from photosystem I at 77K was observed in whole cells and the thylakoid membranes under high light conditions. Interestingly, the chlorophyll fluorescence emission from the photosystem I trimers at 77K are similar to that of the wild type, while the chlorophyll fluorescence from the photosystem I monomers was at a much lower level in the mutant than in the wild type under high light conditions. The RpaA deletion resulted in a dramatic reduction in the monomeric photosystem I and the D1 protein but not the CP47 content. However, there is no significant difference in the transcript levels of psaA or psbA or other genes examined, most of which are involved in photosynthesis, pigment biosynthesis or stress responses. Under high light conditions, the growth of the mutant was affected, and both the chlorophyll content and the whole-chain oxygen evolution capability of the mutant were found to be significantly lower than those of the wild type, respectively. Therefore, RpaA regulates posttranscriptionally the accumulation of the monomeric photosystem I and the D1 protein under high light conditions. The dspA- mutant grows noticeably slower than the wild type under low light conditions, and it loses its viability upon prolonged exposure to high light. The DspA was found responsible for downregulation of photosystem I under high light conditions. It was also found to be responsible for the accumulation and stability of phosystem II under both low light and high light conditions. These studies demonstrated that both RpaA and DspA are important for growth, photosynthesis, acclimation, and survival of cells under high light conditions.

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