Abstract
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 BLUF domain, controls the biofilm formation of the purple photosynthetic bacterium, Rhodopseudomonas palustris. Light-excited PapB enhances the phosphodiesterase activity of the EAL-domain protein, PapA, which degrades the second messenger, cyclic dimeric GMP (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 alpha-helices of the PapB BLUF domain. Thus, the conformational change, which involves the C-terminal alpha-helices, transfers the flavin-sensed blue-light signal to PapA. Deletion of papB in R. palustris enhances biofilm formation under blue light conditions. These results demonstrate that R. palustris can control biofilm formation via a blue light-dependent degradation of c-di-GMP by PapB.
