Structure of a Bacteriophytochrome and Light-Stimulated Protomer Swapping with a Gene Repressor

SummaryPhytochromes are photoreceptors in phototropic organisms that respond to light conditions by changing interactions between a response regulator and DNA. Bacterial phytochromes (BphPs) comprise an input photosensory core domain (PCD) and an output transducing domain (OTD). We report the structure of a BphP containing both PCD and the majority of its OTD, and demonstrate interaction with its cognate repressor. The OTD of RpBphP1, from Rhodopseudomonas palustris, is composed of a PAS/PAC domain and, to our knowledge, a hitherto unrecognized two-helix output sensor (HOS) domain. Unlike canonical BphPs, it does not transmit phosphorelay signals but forms a complex with the transcriptional repressor RpPpsR2 on photoconversion with far-red light. We show that HOS is essential for complex formation and that the anti-parallel dimer geometry is crucial in achieving HOS domain activation and protomer swapping under the control of light. These results provide insights into the steps taken by a two-component signaling system.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (98 K)Download as PowerPoint slideHighlights► Bacteriophytochrome structure containing photosensory and output transducing domain ► RpBphP1 binds to the cognate repressor RpPpsR2 on illumination with far-red light ► HOS domain is distantly related to Dhp and is essential for binding to the repressor ► Light-induced protomer swapping mechanism between dimers