A Mn-sensing riboswitch activates expression of a Mn(2+)/Ca(2+) ATPase transporter in Streptococcus

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(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.