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A homologue of type I rhodopsin was found in the unicellular  Gloeobacter violaceus  PCC7421, which is believed to be primitive because of the lack of thylakoids and peculiar morphology of phycobilisomes. The  Gloeobacter  rhodopsin (GR) gene encodes a polypeptide of 298 amino acids. This gene is localized alone in the genome unlike cyanobacterium  Anabaena  opsin, which is clustered together with 14 kDa transducer gene. Amino acid sequence comparison of GR with other type I rhodopsin shows several conserved residues important for retinal binding and H +  pumping. In this study, the gene was expressed in  Escherichia coli  and bound all- trans  retinal to form a pigment (λmax  = 544 nm at pH 7). The pK a  of proton acceptor (Asp121) for the Schiff base, is approximately 5.9, so GR can translocate H +  under physiological conditions (pH 7.4). In order to prove the functional activity in the cell, pumping activity was measured in the sphaeroplast membranes of  E. coli  and one of  Gloeobacter  whole cell. The efficient proton pumping and rapid photocycle of GR strongly suggests that  Gloeobacter  rhodopsin functions as a proton pumping in its natural environment, probably compensating the shortage of energy generated by chlorophyll-based photosynthesis without thylakoids.

Cyanobacterial Light-Driven Proton Pump, Gloeobacter Rhodopsin: Complementarity between Rhodopsin-Based Energy Production and Photosynthesis