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Maria del Carmen Santos Merino (Ducat lab)

Date & Location: January 28, 2020, at 12p; Room 168 Plant Biology Building

Subject: Testing the capacity of heterologous metabolic sinks to replace photoprotective mechanisms in cyanobacteria

Abstract: Maximizing photosynthetic efficiency is a grand challenge goal for engineering cyanobacteria and may be critical to the economic viability of bioproduction from these microbes. Cyanobacteria possess energy dissipation mechanisms that use to protect themselves under high light and/or fluctuating light conditions, which inherently reduce photosynthetic efficiency. Among them, alternative terminal oxidases (RTOs) and flavodiiron proteins (FDPs) are primary mechanisms that dissipate a large proportion of their excess light energy. Numerous observations of different cyanobacterial species suggest that significant increases in photosynthetic efficiency output can be achieved by engineering metabolic sink capacity. We consider if heterologous engineered pathways (sucrose production and/or cytochrome P450 enzymes) can be used to partially replace these endogenous protective mechanisms, instead conserving captured energy in “useful” metabolic pathways.

To explore this hypothesis, we have constructed mutants lacking FDPs (Δflv3/flv1) and/or blocked RTOs (Cox and Cyd) in Synechococcus elongatus PCC 7942, and introduced heterologous pathways that can act as carbon or electron sinks (sucrose export and/or cytochrome P450s, respectively). The results of these experiments showed additive effects of engaging both sinks, indicating that neither sink alone is capable of utilizing the full “over-potential” of the electron transport chain. However, there are indications of diminishing returns with additional sinks, and we find evidence that there is clear competition between two heterologous sinks in the form of reduced cytochrome P450 activity when the sucrose sink is also engaged. The combination of both systems has a slight additive effect in the electron transport rate and the enhancement appears to depend on light intensity. Conversely, heterologous sinks may partially compensate for the loss of the photoprotective mechanisms (FDPs) even under dynamic (fluctuating) light, but this compensation is inefficient.

Taken together, our results suggest the activity of heterologous sinks may be enhanced by the removal of endogenous photoprotective mechanisms, effectively converting “wasted” energy into useful biological work. Yet, significant improvements in the capacity for engineered metabolic sinks to oxidize the electron transport chain must be achieved to compensate for the loss of native photoprotective pathways.

Speaker Lab: Dr. Danny Ducat