Metabolic Sensing of Extra-cytoplasmic Copper Availability via Translational Control by a Nascent Exported Protein

Abstract

Metabolic sensing is a crucial prerequisite for cells to adjust their physiology to rapidly changing environments. In bacteria, the response to intra- and extra-cellular ligands is primarily controlled by transcriptional regulators, which activate or repress gene expression to ensure metabolic acclimation. Translational control, such as ribosomal stalling can also contribute to cellular acclimation and has been shown to mediate responses to changing intracellular molecules. In the current study, we demonstrate that co-translational export of the protein CutF regulates translation of the down-stream cutO-encoded multi-copper oxidase CutO in response to extracellular copper (Cu). Our data show that CutF, acting as a Cu sensor, is co-translationally exported by the signal recognition particle pathway. Binding of Cu to the periplasmically exposed Cu-binding motif of CutF delays its co-translational export via its C-terminal ribosome stalling-like motif. This allows the unfolding of an mRNA stem-loop sequence that shields the ribosome-binding site of cutO, which favors its subsequent translation. Bioinformatic analyses reveal that CutF-like proteins are widely distributed in bacteria and often, are located upstream of genes involved in transition metal homeostasis. Our overall findings illustrate a highly conserved control mechanism using co-translational export of a protein acting as a sensor to integrate the changing availability of extracellular nutrients into metabolic acclimation. ImportanceMetabolite sensing is a fundamental biological process, and the perception of dynamic changes in the extracellular environment is of paramount importance for the survival of organisms. Bacteria usually adjust their metabolism to changing environments by transcriptional regulation. Here, we describe an alternative translational mechanism that controls the bacterial response to the presence of copper, a toxic micronutrient. This mechanism involves a co-translationally secreted protein that, in the presence of copper, undergoes a process resembling ribosomal stalling. This allows the unfolding of a downstream mRNA stem-loop and enables translation of the adjacent Cu-detoxifying multicopper oxidase. Bioinformatic analyses reveal that such proteins are widespread, suggesting that metabolic sensing using ribosome-arrested nascent secreted proteins acting as sensors may be a common strategy for integrating environmental signals into metabolic adaptation.