Substrate binding in vitro and kinetics of RsrI [N6-adenine] DNA methyltransferase

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Restriction-modification systems are crucial for bacterial defense and DNA regulation.
• DNA methyltransferases play key roles in these systems by modifying specific DNA sequences.
• Understanding the mechanism of DNA methyltransferases like M.RSR:I provides insights into DNA-protein interactions.

Purpose of the Study:

• To purify and characterize the N:6-adenine DNA methyltransferase (M.RSR:I) from Rhodobacter sphaeroides.
• To investigate the DNA binding properties and catalytic mechanism of M.RSR:I.
• To determine the effect of DNA methylation status and cofactor analogs on enzyme activity.

Main Methods:

• Purification of M.RSR:I using ion exchange chromatography.
• Electrophoretic gel retardation assays to study DNA binding.
• Fluorescence spectroscopy to monitor base extrusion.
• Pre-steady-state kinetic and isotope-partitioning experiments to elucidate the reaction mechanism.
• Fluorescence-quenching assays to determine dissociation constants.

Main Results:

• M.RSR:I was purified to >95% homogeneity.
• The enzyme exhibited preferential binding to hemimethylated DNA over unmethylated or dimethylated DNA in the presence of sinefungin (AdoMet analog).
• Evidence for target base extrusion (base flipping) was observed through gel retardation and fluorescence assays.
• Kinetic studies indicated burst kinetics and ruled out an ordered DNA binding mechanism.

Conclusions:

• M.RSR:I displays a distinct DNA binding preference based on methylation status.
• The enzyme mechanism involves the extrusion of the target adenine base.
• The catalytic competence of the M.RSR:I-AdoMet complex was confirmed, shedding light on the enzyme's reaction pathway.