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ABSTRACT His-Asp phosphorelay (also known as two-component signal transduction) proteins are the predominant mechanism used in most bacteria to control behavior in response to changing environmental conditions. In addition to systems consisting of a simple two-component system utilizing an isolated histidine kinase/response regulator pair, some bacteria are enriched in histidine kinases that serve as signal integration proteins; these kinases are usually characterized by noncanonical domain architecture, and the responses that they regulate may be difficult to identify. The environmental bacterium Myxococcus xanthus is highly enriched in these noncanonical histidine kinases. M. xanthus is renowned for a starvation-induced multicellular developmental program in which some cells are induced to aggregate into fruiting bodies and then differentiate into environmentally resistant spores. Here, we characterize the M. xanthus orphan hybrid histidine kinase SinK (Mxan_4465), which consists of a histidine kinase transmitter followed by two receiver domains (REC 1 and REC 2 ). Nonphosphorylatable sinK mutants were analyzed under two distinct developmental conditions and using a new high-resolution developmental assay. These assays revealed that SinK autophosphorylation and REC 1 impact the onset of aggregation and/or the mobility of aggregates, while REC 2 impacts sporulation efficiency. SinK activity is controlled by a genus-specific hypothetical protein (SinM; Mxan_4466). We propose that SinK serves to fine-tune fruiting body morphology in response to environmental conditions. IMPORTANCE Biofilms are multicellular communities of microorganisms that play important roles in host disease or environmental biofouling. Design of preventative strategies to block biofilms depends on understanding the molecular mechanisms used by microorganisms to build them. The production of biofilms in bacteria often involves two-component signal transduction systems in which one protein component (a kinase) detects an environmental signal and, through phosphotransfer, activates a second protein component (a response regulator) to change the transcription of genes necessary to produce a biofilm. We show that an atypical kinase, SinK, modulates several distinct stages of specialized biofilm produced by the environmental bacterium Myxococcus xanthus . SinK likely integrates multiple signals to fine-tune biofilm formation in response to distinct environmental conditions.
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Arm-in-Arm Response Regulator Dimers Promote Intermolecular Signal Transduction
ABSTRACT Bacteriophytochrome photoreceptors (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, the R. tataouinensis bacteriophytochrome response regulator (RtBRR), and a homolog, AtBRR from Agrobacterium tumefaciens , crystallize unexpectedly as arm-in-arm dimers, reliant on a conserved hydrophobic motif, hFWAhL (where h is a hydrophobic M, V, L, or I residue). 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 single-wavelength anomalous diffraction phases and may be of general interest. Examination of the effect of the BRR stoichiometry on signal transduction showed that phosphorylated RtBRR is accumulated more efficiently than the engineered monomeric RtBRR (RtBRR mon ) 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 the resultant phenotypes. To begin to address this knowledge gap, we solved the crystal structures of two single-domain response regulators encoded by a region immediately downstream of that encoding 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.
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- Award ID(s):
- 1518160
- PAR ID:
- 10018462
- Date Published:
- Journal Name:
- Journal of Bacteriology
- Volume:
- 198
- Issue:
- 8
- ISSN:
- 0021-9193
- Page Range / eLocation ID:
- 1218 to 1229
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1128/JB.00872-15
