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

Abstract

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 BLUF domain, controls the biofilm formation of the purple photosynthetic bacterium Rhodopseudomonas palustris. Purified PapB undergoes a typical BLUF-type photocycle, and 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 α-helices of the PapB BLUF domain. Thus, the conformational change, which involves the C-terminal α-helices, transfers the flavin-sensed blue light signal to PapA. Deletion of papB 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.

Exploring Environmental Control of Cyclic di-GMP Signaling in Vibrio cholerae by Using theEx VivoLysate Cyclic di-GMP Assay (TELCA)

“…How the extracellular environment impacts the synthesis and degradation of intracellular c-di-GMP is poorly understood. It has been proposed that extracellular stimuli, such as oxygen, light, amino acids, and QS autoinducers, regulate intracellular c-di-GMP by altering the activity of DGCs and PDEs (16)(17)(18)(19)(20)(21)(22)(23)(24). Also, many DGCs and PDEs have domains predicted to be involved in sensing extracellular ligands (54).…”

“…It has been suggested that extracellular factors regulate DGCs and PDEs, altering the intracellular concentration of c-di-GMP to adapt to changing environments. Indeed, some environmental signals, such as light, oxygen, amino acids, quorum-sensing autoinducers, and norspermidine, have been shown to alter intracellular c-di-GMP in various bacteria (16)(17)(18)(19)(20)(21)(22)(23)(24). However, the vast majority of the regulatory inputs that control DGCs and PDEs remain undetermined.…”

Cyclic di-GMP: the First 25 Years of a Universal Bacterial Second Messenger

“…As discussed above (see "Regulation by Sensory Domains"), many enzymes involved in c-di-GMP signaling contain sensory domains and respond to specific signals. Some enzymes respond to environmental signals by interacting with sensor proteins (136,368). At present, a relatively limited number of signals affecting activities of the c-di-GMP-metabolizing proteins have been investigated experimentally.…”

Bile Acids and Bicarbonate Inversely Regulate Intracellular Cyclic di-GMP in Vibrio cholerae

“…One hypothesis is that each DGC or PDE senses and responds to a specific environmental signal by altering c-di-GMP synthesis or degradation activity in the C-terminal domain. While a few environmental cues, including light, oxygen, zinc, arginine, quorum sensing (QS) autoinducers, and norspermidine, have been shown to directly regulate DGCs or PDEs in various bacteria (14,(16)(17)(18)(19)(20)(21)(22)(23)(24), the environmental signals recognized by the vast majority of DGCs and PDEs remain unidentified.…”