Potential of proton-pumping rhodopsins: engineering photosystems into microorganisms

• PPRs are photosystems that can be relatively easily introduced into microorganisms.
• A large natural diversity of PPRs is available for different applications and hosts.
• PPRs allow for some light-driven biotechnological conversions in microorganisms.
• PPRs can potentially power more energy-limited biotechnological production pathways.

A wide range of proton-pumping rhodopsins (PPRs) have been discovered in recent years. Using a synthetic biology approach, PPR photosystems with different features can be easily introduced in nonphotosynthetic microbial hosts. PPRs can provide hosts with the ability to harvest light and drive the sustainable production of biochemicals or biofuels. PPRs use light energy to generate an outward proton flux, and the resulting proton motive force can subsequently power cellular processes. Recently, the introduction of PPRs in microbial production hosts has successfully led to light-driven biotechnological conversions. In this review, we discuss relevant features of natural PPRs, evaluate reported biotechnological applications of microbial production hosts equipped with PPRs, and provide an outlook on future developments.