Reconsidering the czcD (NiCo) Riboswitch as an Iron Riboswitch

Area of Science:

• Molecular Biology
• Biochemistry
• Microbiology

Background:

• Bacterial iron regulation is essential for cellular function.
• Riboswitches are RNA molecules that regulate gene expression in response to small molecules.
• The czcD riboswitch was initially characterized as a nickel-Co(II) riboswitch.

Purpose of the Study:

• To provide direct evidence for iron(II) as a physiological ligand for the czcD riboswitch class.
• To compare the binding affinities and conformational responses of czcD riboswitches to various metal ions.
• To clarify the role of czcD riboswitches in bacterial iron homeostasis.

Main Methods:

• Isothermal titration calorimetry (ITC) was used to study metal ion binding to native czcD riboswitches from three different organisms.
• Fluorescent sensors were previously employed to assess riboswitch conformational changes.

Main Results:

• czcD riboswitches exhibit similar dissociation constants (approximately 1 μM) and conformational responses to iron(II), cobalt(II), and nickel(II).
• Only the iron response occurs within the physiological concentration range found in bacterial cells.
• The riboswitches showed no response to manganese(II) and a weak response to zinc(II).
• The "Sensei" RNA, previously proposed as an iron-specific riboswitch, did not respond to iron(II).

Conclusions:

• Iron responsiveness is a conserved characteristic of the czcD riboswitch family.
• The czcD riboswitch represents the only known family of iron-responsive riboswitches.
• These findings establish the czcD riboswitch as a key player in bacterial iron regulation and warrant further investigation into its physiological functions.