Understanding CupA's role in the CO2 concentrating mechanism in cyanobacteria

Abstract

The CO2 concentrating mechanism (CCM) in cyanobacteria is one of the most unique and efficient systems used to optimize photosynthetic output on the planet. The structure of the CCM in Synechococcus elongatus has recently been resolved, but how the complex works together in order to overcome environmental and physiological obstacles is still mostly unknown. One way cyanobacteria’s CCM works is by using CO2 uptake proteins (Cup) in cooperation with the rest of the NDH-1 complexe on the thylakoid membrane to convert CO2 in the cell to HCO3. When inorganic carbon in the environment exists mostly as CO2, and the amount of overall inorganic carbon is limited, CupA is induced, alongside its constitutively expressed paralog, (CupB). CupA and CupB possess remarkably similar functionality, differing slightly in their expression and kinetic ability. While CupB is constitutively expressed in the cell, it is shown to have substantially lower affinity for CO2. CupA’s increased affinity for CO2, compared to CupB, is particularly useful when CO2 is limited in the environment. However, these two Cup proteins possess a very intriguing, and largely unknown, relationship with each other. Previous work suggests that CupA may only work with CupB, while the reverse is not true as CupB works efficiently with or without CupA. This research is to test this hypothesis, and to better determine the relationship between these proteins. This was done by genetically modifying strains of Synechococcus sp. PCC7942 for comparison by physiological assays.