DNA synapsis through transient tetramerization triggers cleavage by Ecl18kI restriction enzyme

Area of Science:

• Molecular Biology
• Enzymology
• Structural Biology

Background:

• Restriction enzymes are crucial for DNA manipulation, forming various oligomeric structures to bind and cleave target DNA sequences.
• The endonuclease Ecl18kI exists as a dimer in solution but crystallizes as a tetramer bound to DNA.

Purpose of the Study:

• To determine the functional oligomeric state of Ecl18kI during DNA cleavage.
• To investigate the role of the dimer-dimer interface in Ecl18kI activity and DNA interaction.

Main Methods:

• Atomic force microscopy (AFM) to visualize DNA looping and tetramer formation on DNA.
• Tethered particle motion (TPM) to study the dynamics of DNA looping in solution.
• Site-directed mutagenesis to disrupt the putative dimer-dimer interface (e.g., R174A mutant).
• Functional assays to assess DNA cleavage activity of wild-type and mutant enzymes.

Main Results:

• Ecl18kI forms a tetramer and loops DNA on two-site substrates, as observed by AFM.
• DNA looping mediated by Ecl18kI is a dynamic process involving transient tetramer formation in solution, shown by TPM.
• Ecl18kI exhibits significantly faster DNA cleavage in the tetrameric synaptic complex compared to single-site interactions.
• The R174A mutant, unable to tetramerize, binds DNA as a dimer, fails to loop DNA, and shows minimal cleavage activity.

Conclusions:

• Ecl18kI utilizes an association model for synaptic complex assembly, initially binding DNA as a dimer.
• Transient tetramerization of Ecl18kI is essential for DNA looping and efficient catalytic cleavage.
• The dimer-dimer interface is critical for the functional tetramer formation and the enzyme's overall activity.