A Mn-sensing riboswitch activates expression of a Mn2+/Ca2+ ATPase transporter in Streptococcus

Abstract

Maintaining manganese (Mn) homeostasis is important for the virulence of numerous bacteria. In the human respiratory pathogen $Streptococcus pneumoniae$, the Mn-specific importer PsaBCA, exporter MntE, and transcriptional regulator PsaR establish Mn homeostasis. In other bacteria, Mn homeostasis is controlled by $yybP-ykoY$ family riboswitches. Here, we characterize a $yybP-ykoY$ family riboswitch upstream of the $mgtA$ gene encoding a P$_{\rm{II}}$-type ATPase in $S. pneumoniae$, suggested previously to function in Ca$^{2+}$ efflux. We show that the $mgtA$ riboswitch aptamer domain adopts a canonical $yybP-ykoY$ structure containing a three-way junction that is compacted in the presence of Ca$^{2+}$ or Mn$^{2+}$ at a physiological Mg$^{2+}$ concentration. Although Ca$^{2+}$ binds to the RNA aptamer with higher affinity than Mn$^{2+}$, in vitro activation of transcription read-through of $mgtA$ by Mn$^{2+}$ is much greater than by Ca$^{2+}$. Consistent with this result, $mgtA$ mRNA and protein levels increase ≈5-fold during cellular Mn stress, but only in genetic backgrounds of $S. pneumoniae$ and $Bacillus subtilis$ that exhibit Mn$^{2+}$ sensitivity, revealing that this riboswitch functions as a failsafe ‘on’ signal to prevent Mn$^{2+}$ toxicity in the presence of high cellular Mn$^{2+}$. In addition, our results suggest that the $S. pneumoniae$ $yybP-ykoY$ riboswitch functions to regulate Ca$^{2+}$ efflux under these conditions.