Intermolecular contacts between the lambda-Cro repressor and the operator DNA characterized by nuclear magnetic...

Area of Science:

• Molecular Biology
• Structural Biology
• Biophysics

Background:

• Lambda phage Cro repressor is a key regulator of viral gene expression.
• Understanding protein-DNA interactions is crucial for molecular biology.
• The Cro repressor binds to operator DNA sequences to control transcription.

Purpose of the Study:

• To elucidate the specific interactions between lambda phage Cro repressor and its DNA operator sequence.
• To determine the structural basis of Cro repressor-DNA binding.
• To explain the differential binding affinities of Cro repressor to various operator sequences.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy, including heteronuclear NMR experiments.
• Use of isotopically labeled (15N, 13C/15N) lambda-Cro protein.
• Heteronuclear filtered NOESY experiments to identify inter-molecular contacts.
• Restrained molecular dynamics simulations to dock the protein to DNA.

Main Results:

• Detailed chemical shift assignments of the lambda-Cro-DNA complex were obtained.
• Inter-molecular contacts were identified between the Cro repressor's helix-turn-helix motif and the DNA major groove.
• Specific hydrophobic contacts were observed between amino acid residues (Thr17, Val25, Ala29) and thymine methyl groups in the DNA.
• The structure of Cro repressor in complex is similar to its unbound form.

Conclusions:

• The helix-turn-helix region of lambda Cro repressor directly interacts with the DNA major groove.
• Specific hydrophobic contacts are critical determinants of Cro repressor binding affinity to operator DNA.
• These findings provide a structural basis for understanding sequence-specific DNA recognition by repressors.