Functional Expression of Rhodopsin in PSI-Less sp. PCC6803.

The approach of providing an oxygenic photosynthetic organism with a cyclic electron transfer system, i.e., a far-red light-driven proton pump, is widely proposed to maximize photosynthetic efficiency via expanding the absorption spectrum of photosynthetically active radiation. As a first step in this approach, rhodopsin was expressed in a PSI-deletion strain of sp. PCC6803. Functional expression of rhodopsin, in contrast to Proteorhodopsin, did not stimulate the rate of photoheterotrophic growth of this strain, analyzed with growth rate measurements and competition experiments. Nevertheless, analysis of oxygen uptake and-production rates of the rhodopsin-expressing strains, relative to the ΔPSI control strain, confirm that the proton-pumping rhodopsin provides the cells with additional capacity to generate proton motive force. Significantly, expression of the rhodopsin did modulate levels of pigment formation in the transgenic strain.