Advanced Macular Degeneration is the current leading cause of blindness and vision loss in people aged 65 and over. 1.75 million people in the US are affected by AMD, and that number is expected to increase to almost 200 million worldwide by 2020. Lutein is a carotenoid currently used as a dietary supplement taken to treat and prevent the onset of AMD. The production of lutein is currently done through the cultivation of marigolds. Carotenoids are extracted from its petals, from which lutein is isolated.
Our aim is to introduce a system capable of producing lutein from carotenoid precursors into a bacterial system. As these pathways are native to plants and nonexistent in bacteria, the main challenge is obtaining every enzyme necessary to allow the pathway to occur. Strains of lycopene (a carotenoid precursor) producing bacteria already exist, and we expect to begin the synthesis from this point. Another main challenge is the regulation of genes required to proceed from lycopene to lutein. Lycopene is the precursor for a multitude of carotenoids, all of which are produced in plants due to necessity. To produce lutein, the lycopene must first undergo a reaction with the specific enzymes in order, ε-cyclase then β-cyclase. Having both enzymes in the system, though, allows reactions between β-cyclase and lycopene, which yields a product unable to be converted into lutein. Through regulatory measures such as altering gene expression levels, we plan to optimize the efficiency of the lutein synthesis pathway. We also aim to create a mathematical modeling system capable of correlating the expression levels of proteins to relevant efficiencies in similar synthesis pathways.
Antibiotic resistance is a necessary selection factor for transgenic bacteria using plasmids as vectors. This staple of genetic engineering has been met with opposition with valid claims that the addition of antibiotics to the environment harms native species and poses a risk to unwanted antibiotic resistance through lateral gene transfer.
The Hok/Sok system has naturally evolved in bacteria as a means of plasmid retention, and is capable of addressing the issue by providing a selection factor for plasmid retention without the dangers of antibiotics and risk of lateral gene transfer. The Hok (host killing) gene codes for a mRNA which lies dormant in its initial secondary structure. As it is degraded by exonuclease, it assumes a translatable secondary structure which produces an apoptosis triggering protein. The Sok (suppression of killing) gene codes for a mRNA transcript that binds to the Hok mRNA, preventing it from being translated. The complex is eventually degraded by nuclease. Hok has a half life of 20 minutes, while Sok has a half life of 30 seconds. As long as both genes are present, the cell remains alive. After cell division, should the cell not retain the plasmid of interest which contains Hok/Sok, Hok mRNA remains the cytoplasm for 20 minutes, while remaining Sok is degraded. Since the cell does not contain a Sok gene, no Sok is being produced to save the cell from being killed by Hok. This system is very similar to current antibiotic resistance systems, only without the necessity for antibiotics themselves, resolving the issue of environmentally safe plasmid retention.
