Magnesium controls aptamer-expression platform switching in the SAM-I riboswitch

Area of Science:

• Molecular Biology
• Biophysics
• Gene Regulation

Background:

• Riboswitches regulate gene transcription through ligand-binding aptamer domains.
• The aptamer and expression platform domains compete for a shared strand, determining transcription ON/OFF states.
• Understanding the full-length riboswitch dynamics and environmental influences is crucial for comprehending gene regulation.

Purpose of the Study:

• To investigate the ON-OFF transition mechanism of the SAM-I riboswitch.
• To determine the dependence of this transition on magnesium concentration.
• To elucidate the role of magnesium in modulating riboswitch-mediated transcription.

Main Methods:

• Expression platform switching experimental assay.
• Structure-based electrostatic simulations.
• Analysis of magnesium concentration effects on riboswitch populations.

Main Results:

• The ratio of OFF to ON states in the SAM-I riboswitch varies non-monotonically with magnesium concentration.
• Low to intermediate magnesium concentrations favor the OFF state by pre-organizing the aptamer domain.
• Higher magnesium concentrations stabilize the anti-terminator helix, favoring the ON state and destabilizing the OFF state via domain closure.

Conclusions:

• Magnesium plays a critical role in controlling gene transcription by modulating riboswitch function.
• A specific, narrow magnesium concentration range is essential for balancing OFF and ON states in bacterial gene regulation.
• The findings highlight the functional significance of magnesium in physiological gene expression.