A c-di-AMP riboswitch controlling kdpFABC operon transcription regulates the potassium transporter system in Bacillus thuringiensis

The intracellular K(+) level in bacteria is strictly controlled by K(+) uptake and efflux systems. Among these, KdpFABC is a high-affinity K(+) transporter system that is generally activated by the KdpDE two-component system in response to K(+) limitation stress. However, the regulatory mechanism remains obscure in bacteria lacking the kdpDE genes. Here we report that the transcription of a kdpFABC operon is distinctively regulated by a cyclic diadenylate monophosphate (c-di-AMP) riboswitch located at the 5′-untranslated region of kdp transcript, and binding of c-di-AMP to the riboswitch promotes its intrinsic termination that blocks the kdpFABC transcription. Further, the intracellular c-di-AMP concentration was found to decrease under the K(+) limitation stress, leading to transcriptional read-through over the terminator to allow kdpFABC expression. This regulatory element is found predominantly in the Bacillus cereus group and correlate well with the K(+) and c-di-AMP homeostasis that affects a variety of crucial cellular functions.