Advancing the cyanobacterial cell factory: Synechocystis sp. 6803

Cyanobacteria are photoautotrophic prokaryotes capable of converting light into chemical energy via oxygenic photosynthesis. This chemical energy is conventionally used to fuel growth and the generation of progeny however, it has been demonstrated that it can be redirected away from growth and used to generate useful carbon-based compounds such as fuels and pharmaceuticals in a renewable manner.

Synechocystis sp. 6803 has become a model cyanobacterium in this field as it is unicellular, naturally transformable and possess a fully sequenced and annotated genome. Whilst a variety of carbon based compounds have been successfully produced in Synechocystis, the yields are significantly less than those achieved in Escherichia coli or Saccharomyces cerevisiae. The input costs of engineering specific strains, growing them at scale, harvesting biomass and extracting the product of interest significantly outweigh the value of the desired product. A lack of understanding of the basic cellular process that control and regulate gene expression is responsible for these low yields.

This project aims to reveal a more detailed insight into the genome architecture, mechanisms of DNA replication and the regulation of genome copy number in Synechocystis sp. 6803. It is now well understood that Synechocystis is polyploid and the exact number of genome copies per cell fluctuates depending on the growth phase and environmental conditions. However, it is as of yet unknown why the genome copy number fluctuates and how this process is regulated.
Furthermore, the chromosome of Synechocystis has an as of yet undescribed origin of replication and some studies in this area have suggested that the DNA replication machinery thought to be ubiquitous to prokaryotes is not essential in Synechocystis. And finally, it is assumed that DNA in prokaryotes is located freely within the cytosol and that genome organisation is a phenomenon reserved only for eukaryotes; an assumption that has yet to be confirmed.

Simply constitutively expressing heterologous metabolic pathways has only resulted in genetic instability and a metabolic burden on the host cell. Control over heterologous gene expression is key to achieving high yields of desired products. By revealing the molecular processes by which Synechocystis regulates its genome copy number, initiates DNA replication and organises its genome, this study hopes to provide those working in this field with greater control over how and when heterologous genes are expressed in Synechocystis, thereby improving the notion of a cyanobacterial cell factory.