DNA binding properties of the LexA repressor.

Area of Science:

• Molecular Biology
• Structural Biology
• Genetics

Background:

• The LexA repressor from Escherichia coli regulates transcription of approximately 20 genes.
• LexA binding affinity varies across single, double, or triple operators, with cooperative binding observed under favorable spacing.
• LexA recognizes DNA through its amino-terminal domain.

Purpose of the Study:

• To elucidate the three-dimensional structure of the LexA repressor's amino-terminal DNA-binding domain.
• To characterize the DNA-binding mechanism of LexA and compare it to known DNA-binding proteins.
• To identify regions critical for DNA recognition by LexA.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the 3D structure of the LexA amino-terminal domain.
• Homology considerations and analysis of DNA backbone contact patterns were employed.
• Mutagenesis studies identified LexA mutants deficient in DNA binding.

Main Results:

• The LexA amino-terminal domain comprises three alpha-helices (residues 8-20, 28-35, and 41-54).
• LexA's structure and DNA interaction pattern suggest it possesses a novel DNA-binding motif, differing from canonical helix-turn-helix proteins.
• Mutations within the third helix (residues 41-54) significantly impair DNA binding, implicating this helix in DNA recognition.

Conclusions:

• LexA represents a distinct class of DNA-binding proteins, potentially containing a new DNA-binding motif.
• The third alpha-helix of the LexA amino-terminal domain is essential for its interaction with DNA.
• Further research is warranted to fully characterize this novel DNA-binding mechanism.