Characterization of cytochromes and plastocyanin in the cyanobacterium Synechocystis sp. PCC 6803

The main objective of this project is to study the components involved in electron transport between the cytochrome bf complex and photosystem I in the unicellular cyanobacterium Synechocystis 6803. Two soluble proteins, the heme-protein cytochrome c553 and the copper protein plastocyanin, were isolated from this species and specific antibodies were raised to each protein. Using antibodies as probes, the reciprocal accumulation of the two proteins in response to the copper concentration in the medium was characterized. Cytochrome c553 was present when cells were grown in the copper deficient medium, while plastocyanin was present when cells were provided with sufficient copper. The gene of cytochrome c553 was cloned and sequenced from Synechocystis 6803. The 360 bp open-reading frame encodes a 35 amino acid pre-sequence and a 85 amino acid mature protein. Northern analysis indicates that regulation of the cytochrome c553 gene expression by copper is at a stage prior to translation and that the transcript is a monocistronic mRNA. Mutants were constructed in which the gene for cytochrome c553 was deleted and the gene for plastocyanin was interrupted by insertional inactivation. When the synthesis of plastocyanin was inhibited by copper deprivation in the cytochrome c553 deletion mutant, or when the synthesis of cytochrome c553 was inhibited by the presence of copper in plastocyanin deficient mutant, both mutant strains exhibited normal photoautotrophic growth and steady-state photosynthetic electron transport rates. These observations indicate that electrons are transferred from the cytochrome bf complex to photosystem I in the absence of both cytochrome c553 and plastocyanin. Analysis of cytochrome composition did not reveal any induced cytochrome component that could substitute for the role of plastocyanin or cytochrome c553 in the mutant strains. However, we have evidence that there is an unknown membrane bound redox component with an absorption peak near 560 nm in an ascorbate minus hydroquinone chemical difference spectrum that may be a b-type cytochrome.U of I OnlyETDs are only available to UIUC Users without author permissio