Suppressor mutations reveal an NtrC-like response regulator, NmpR, for modulation of Type-IV Pili-dependent motility in Myxococcus xanthus

Cell-cell transfer of adaptation traits benefits kin and actor in a cooperative microbe

Microbes face many physical, chemical, and biological insults from their environments. In response, cells adapt, but whether they do so cooperatively is poorly understood. Here, we use a model social bacterium, Myxococcus xanthus, to ask whether adapted traits are transferable to naïve kin. To do so we isolated cells adapted to detergent stresses and tested for trait transfer. In some cases, strain-mixing experiments increased sibling fitness by transferring adaptation traits. This cooperative behavior depended on a kin recognition system called outer membrane exchange (OME) because mutants defective in OME could not transfer adaptation traits. Strikingly, in mixed stressed populations, the transferred trait also benefited the adapted (actor) cells. This apparently occurred by alleviating a detergent-induced stress response in kin that otherwise killed actor cells. Additionally, this adaptation trait when transferred also conferred resistance against a lipoprotein toxin delivered to targeted kin. Based on these and other findings, we propose a model for stress adaptation and how OME in myxobacteria promotes cellular cooperation in response to environmental stresses.

A miniTurbo-based proximity labeling protocol to identify conditional protein interactomes in vivo in Myxococcus xanthus

Protein-protein interactions are foundational for many cellular processes. Such interactions are especially challenging to identify if they are transient or depend on environmental conditions. This protocol details steps to identify stable and transient protein interactomes in the bacterium Myxococcus xanthus using biotin ligase miniTurbo-based proximity labeling. We include instructions for optimizing the expression of control proteins, in vivo biotin labeling of bacteria grown on a surface or in suspension culture, enrichment of biotinylated proteins, and sample processing for proteomic analysis. For complete details on the use and execution of this protocol, please refer to Branon et al. (2018).1.

Evolutionary innovation through transcription factor rewiring in microbes is shaped by levels of transcription factor activity, expression, and existing connectivity

The survival of a population during environmental shifts depends on whether the rate of phenotypic adaptation keeps up with the rate of changing conditions. A common way to achieve this is via change to gene regulatory network (GRN) connections-known as rewiring-that facilitate novel interactions and innovation of transcription factors. To understand the success of rapidly adapting organisms, we therefore need to determine the rules that create and constrain opportunities for GRN rewiring. Here, using an experimental microbial model system with the soil bacterium Pseudomonas fluorescens, we reveal a hierarchy among transcription factors that are rewired to rescue lost function, with alternative rewiring pathways only unmasked after the preferred pathway is eliminated. We identify 3 key properties-high activation, high expression, and preexisting low-level affinity for novel target genes-that facilitate transcription factor innovation. Ease of acquiring these properties is constrained by preexisting GRN architecture, which was overcome in our experimental system by both targeted and global network alterations. This work reveals the key properties that determine transcription factor evolvability, and as such, the evolution of GRNs.

Characterization of a Cobalt-Substituted Globin-Coupled Oxygen Sensor Histidine Kinase from Anaeromyxobacter sp. Fw109-5: Insights into Catalytic Regulation by Its Heme Coordination Structure

Heme-based gas sensors are an emerging class of heme proteins. AfGcHK, a globin-coupled histidine kinase from Anaeromyxobacter sp. Fw109-5, is an oxygen sensor enzyme in which oxygen binding to Fe(II) heme in the globin sensor domain substantially enhances its autophosphorylation activity. Here, we reconstituted AfGcHK with cobalt protoporphyrin IX (Co-AfGcHK) in place of heme (Fe-AfGcHK) and characterized the spectral and catalytic properties of the full-length proteins. Spectroscopic analyses indicated that Co(III) and Co(II)-O₂ complexes were in a 6-coordinated low-spin state in Co-AfGcHK, like Fe(III) and Fe(II)-O₂ complexes of Fe-AfGcHK. Although both Fe(II) and Co(II) complexes were in a 5-coordinated state, Fe(II) and Co(II) complexes were in high-spin and low-spin states, respectively. The autophosphorylation activity of Co(III) and Co(II)-O₂ complexes of Co-AfGcHK was fully active, whereas that of the Co(II) complex was moderately active. This contrasts with Fe-AfGcHK, where Fe(III) and Fe(II)-O₂ complexes were fully active and the Fe(II) complex was inactive. Collectively, activity data and coordination structures of Fe-AfGcHK and Co-AfGcHK indicate that all fully active forms were in a 6-coordinated low-spin state, whereas the inactive form was in a 5-coordinated high-spin state. The 5-coordinated low-spin complex was moderately active-a novel finding of this study. These results suggest that the catalytic activity of AfGcHK is regulated by its heme coordination structure, especially the spin state of its heme iron. Our study presents the first successful preparation and characterization of a cobalt-substituted globin-coupled oxygen sensor enzyme and may lead to a better understanding of the molecular mechanisms of catalytic regulation in this family.