Freshwater lakes harbor complex microbial communities, often dominated by a cosmopolitan lineage of actinobacteria called acI. Based on single-cell genome and metagenome sequences, it is thought that members of acI might bolster heterotrophic growth using phototrophy. Ultimately, we seek to determine whether acI indeed takes advantage of the potential light-harvesting capability encoded in its actino-opsin genes (actR), and to understand its nutrient acquisition pathways. Heterologous expression of acI genes predicted to be crucial for the actinorhodopsin pathway produced lycopene, retinal, and ActR. ActR expressed in E. coli forms an active rhodopsin which pumps protons in response to light. A second genome-based prediction is that acI may use the cyanobacterial storage polymer cyanophycin as a nitrogen source. Our biochemical and structural results refute this idea. It remains to be discovered the extent to which rhodopsin plays a role in the metabolism of acI in its natural environment, but the recognition that freshwater bacteria may use this ancient form of light harvesting has opened new ways to consider carbon and energy cycling in Earth’s fresh water.
Time permitting, I’ll share a short unpublished story about the role of a unique four-crossover knot in a second class of photoactive protein, the red light photoreceptor Phytochrome.