Expression of holo-proteorhodopsin in Synechocystis sp. PCC 6803

• Here we show that proteo-opsin can be expressed up to levels of 105 molecules per cell in membranes of Synechocystis sp. PCC6803.
• The heterologously expressed protein is equally distributed (based on amount of protein) over thylakoid- and cytoplasmic membranes.
• Synechocystis sp. PCC6803 is able to synthesize all-trans retinal and generate proteorhodopsin without requiring to add external retinal.
• Proteorhodopsin expressed in Synechocystis sp. PCC6803 can make a measurable contribution to light-energy conversion in this cyanobacterium.

Retinal-based photosynthesis may contribute to the free energy conversion needed for growth of an organism carrying out oxygenic photosynthesis, like a cyanobacterium. After optimization, this may even enhance the overall efficiency of phototrophic growth of such organisms in sustainability applications. As a first step towards this, we here report on functional expression of the archetype proteorhodopsin in Synechocystis sp. PCC 6803. Upon use of the moderate-strength psbA2 promoter, holo-proteorhodopsin is expressed in this cyanobacterium, at a level of up to 105 molecules per cell, presumably in a hexameric quaternary structure, and with approximately equal distribution (on a protein-content basis) over the thylakoid and the cytoplasmic membrane fraction. These results also demonstrate that Synechocystis sp. PCC 6803 has the capacity to synthesize all-trans-retinal. Expressing a substantial amount of a heterologous opsin membrane protein causes a substantial growth retardation Synechocystis, as is clear from a strain expressing PROPS, a non-pumping mutant derivative of proteorhodopsin. Relative to this latter strain, proteorhodopsin expression, however, measurably stimulates its growth.