P22 c2 repressor-operator complex: mechanisms of direct and indirect readout

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• The P22 c2 repressor protein (P22R) regulates the lysogenic pathway of bacteriophage P22.
• Understanding protein-DNA interactions is key to deciphering gene regulation.

Purpose of the Study:

• To elucidate the structural basis of sequence-specific DNA binding by the P22 repressor protein (P22R).
• To investigate the mechanisms of direct and indirect readout in protein-DNA recognition.

Main Methods:

• X-ray crystallography was used to determine the structure of the P22R N-terminal domain (NTD) complexed with a synthetic DNA operator (DNA9T).
• Analysis of protein-DNA interactions, including van der Waals forces and hydrogen bonding.
• Investigation of DNA conformational changes induced by protein binding.

Main Results:

• The 1.6 Å X-ray structure revealed direct readout through van der Waals interactions, particularly a valine residue fitting into a DNA-intrinsic binding cleft.
• Sequence-specificity is primarily mediated by this lock-and-key interaction, independent of protein-induced DNA conformational changes.
• Indirect readout involves protein-induced transition of DNA to a B'-DNA state, influencing affinity through sequence-dependent free energy changes and hydration patterns.

Conclusions:

• P22R employs a dual mechanism for DNA recognition: direct readout via specific steric and van der Waals interactions, and indirect readout through modulation of DNA conformation and hydration.
• The findings provide a structural basis for understanding P22 repressor function and offer insights into general principles of sequence-specific protein-DNA recognition.