Repeat of a helix-turn-helix module in DNA-binding proteins

Area of Science:

• Structural Biology
• Molecular Biology
• Genetics

Background:

• The helix-turn-helix (HTH) motif is a common DNA-binding structure in proteins.
• It is generally assumed that HTH motifs appear once in prokaryotic transcriptional repressors.
• Seven prokaryotic repressors with determined 3-D structures were analyzed.

Purpose of the Study:

• To investigate the modularity of helix-turn-helix motifs in prokaryotic transcriptional repressors.
• To explore the structural organization and potential for multiple HTH motifs within these proteins.
• To propose a basis for further classification of DNA-binding proteins based on HTH module organization.

Main Methods:

• X-ray crystallography was used to determine the 3-D structures of seven prokaryotic repressors.
• Proteins were decomposed into structural units termed 'modules', specifically helix-turn-helix modules.
• Tertiary structures of identified modules were compared.

Main Results:

• Prokaryotic repressors can be decomposed into helix-turn-helix modules and connectors.
• Each HTH motif corresponds to a module of approximately 13 amino acids.
• Three of the seven analyzed repressors contain more than one HTH-like module.
• Differences in module organization suggest varied structural repertoires and DNA recognition capabilities.

Conclusions:

• Prokaryotic repressors exhibit modular organization beyond a single helix-turn-helix motif.
• Multiple HTH modules contribute to structural diversity and functional specialization in DNA-binding proteins.
• The size of a specific side chain influences DNA base recognition specificity within HTH modules.