Towards Synthetic Phototrophy: Engineering Proton-Pumping Rhodopsins into E. coli

Light-driven, microbial production of biochemicals and biofuels is an interesting alternative to fossil resource-based production. However, non-photosynthetic microorganisms are often better characterized and more suitable for metabolic engineering than natural photosynthetic microorganisms. Hence, we aim to develop synthetic phototrophic microorganisms by introducing photosystems into non-photosynthetic, microbial hosts.

The photosystem we focus on is the family of proton-pumping rhodopsins (PPRs) [1]. A wide range of these PPRs have recently been discovered in nature. All these PPRs are transmembrane proteins harbouring a retinal pigment. PPRs are simple photosystems that can easily be introduced in non-photosynthetic hosts. PPRs generally use light energy to generate an outward proton flux, and the resulting proton motive force can subsequently generate ATP. This light-generated ATP can be used to boost ATP-consuming production pathways or CO₂ fixation cycles. Recently, the introduction of PPRs in microbial production hosts, such as E. coli, has successfully led to some light-driven biotechnological conversions.

In our research, the model bacterium E. coli is the host of interest for establishing PPR photosystems and light-driven metabolic production pathways. We focus on two main aspects of PPR-based phototrophy in E. coli: (i) optimizing expression of different PPR photosystems including pigment biosynthesis, and (ii) application of the PPR-generated proton gradients to drive production pathways or CO₂ fixation cycles. We use an integrated experimental and genome-scale modelling approach to investigate the potential of PPRs to boost metabolic pathways in E. coli. Development of a synthetic phototrophic E. coli is a promising development towards a useful photosynthetic, microbial production host.  

1         Claassens, N.J. et al. (2013) Potential of proton-pumping rhodopsins: engineering photosystems into microorganisms. Trends Biotechnol. 31, 633–42