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Publication Abstract from PubMed

Bacteriophytochrome photosensors (BphPs) and their cognate response regulators make up two-component signal transduction systems which direct bacteria to mount phenotypic responses to changes in environmental light quality. Most of these systems utilize single-domain response regulators to transduce signals through unknown pathways and mechanisms. Here we describe the photocycle and autophosphorylation kinetics of RtBphP1, a red light regulated histidine kinase from the desert bacterium Ramlibacter tataouinensis. RtBphP1 undergoes red to far-red photoconversion with rapid thermal reversion to the dark state. RtBphP1 is autophosphorylated in the dark; this activity is inhibited under red light. The RtBphP1 cognate response regulator, RtBRR, and a homolog, AtBRR from Agrobacterium tumefaciens, crystallize unexpectedly as arm-in-arm dimers, reliant on a conserved hydrophobic motif hFWAhL. RtBRR and AtBRR dimerize distinctly from four structurally characterized phytochrome response regulators found in photosynthetic organisms, and from all other receiver domain homodimers in the Protein Data Bank. A unique cacodylate-zinc-histidine tag metal organic framework yielded SAD phases and may be of general interest. Examination of the effect of BRR stoichiometry on signal transduction showed that phosphorylated RtBRR is accumulated more efficiently than the engineered monomeric RtBRRmon in phosphotransfer reactions. Thus, we conclude that arm-in-arm dimers are a relevant signaling intermediate in this class of two component regulatory systems. IMPORTANCE: BphP histidine kinases and their cognate response regulators comprise widespread red light sensing two-component systems. Much work on BphPs has focused on structural understanding of light sensing, and on enhancing the natural infrared fluorescence of these proteins, rather than on signal transduction or resultant phenotypes. To begin to address this knowledge gap we solved the crystal structures of two single domain response regulators encoded immediately downstream of BphPs. We observed a previously unknown "arm-in-arm" dimer linkage. Monomerization via deletion of the C-terminal dimerization motif had an inhibitory effect on net response regulator phosphorylation, underlining the importance of these unusual dimers for signal transduction.

Arm-in-arm response regulator dimers promote intermolecular signal transduction.,Baker AW, Satyshur KA, Morales NM, Forest KT J Bacteriol. 2016 Feb 1. pii: JB.00872-15. PMID:26833410^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.