A eukaryotic BLUF domain mediates light-dependent gene expression in the purple bacterium Rhodobacter sphaeroides 2.4.1.

The flavin-binding BLUF domain functions as a blue-light receptor in eukaryotes and bacteria. In the photoreceptor protein photo-activated adenylyl cyclase (PAC) from the flagellate Euglena gracilis, the BLUF domain is linked to an adenylyl cyclase domain. The PAC protein mediates a photophobic response. In the AppA protein of Rhodobacter sphaeroides, the BLUF domain is linked to a downstream domain without similarity to known proteins. AppA functions as a transcriptional antirepressor, controlling photosynthesis gene expression in the purple bacterium R. sphaeroides in response to light and oxygen. We fused the PACalpha1-BLUF domain from Euglena to the C terminus of AppA. Our results show that the hybrid protein is fully functional in light-dependent gene repression in R. sphaeroides, despite only approximately 30% identity between the eukaryotic and the bacterial BLUF domains. Furthermore, the bacterial BLUF domain and the C terminus of AppA can transmit the light signal even when expressed as separated domains. This finding implies that the BLUF domain is fully modular and can relay signals to completely different output domains.