Biochemical and Physiological Characterization of a BLUF Protein-EAL Protein Complex Involved in Blue Light-Dependent Degradation of Cyclic Diguanylate in the Purple Bacterium Rhodopseudomonas palustris

Organisms adapt their physiologies in response to the quality and quantity of environmental light. Members of a recently identified photoreceptor protein family, BLUF domain proteins, use a flavin chromophore to sense blue light. Herein, we report that PapB, which contains a BM: domain, controls the biofilm formation of the purple photosynthetic bacterium Rhodopseudomonas palustris. Purified Pupil undergoes a typical BLUF-type photocycle, and light-excited PapB enhances the phosphodiesterase activity of the EA L domain protein, PapA, which degrades the second messenger, cyclic dimeric GM P (c-di-GMP). PapB directly interacts with PapA in vitro in a light-independent manner and induces a conformational change in the preformed PapA PapB complex. A PapA PapB docking simulation, as well as a site-directed mutagenesis study, identified amino acids partially responsible for the interaction between the PapA EAL domain and the two C-terminal a-helices of the PapB BLUF domain. Thus, the conformational change, which involves the C-terminal a-helices, transfers the flavin-sensed blue light signal to PapA. Deletion of popB in R. palustris enhances biofilm formation under high-intensity blue light conditions, indicating that PapB functions as a blue light sensor, which negatively regulates biofilm formation. These results demonstrate that R. palustris can control biofilm formation via a blue light-dependent modulation of its c-di-GMP level by the BLUF domain protein, PapB.