Localized DNA flexibility contributes to target site selection by DNA-bending proteins

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• DNA-binding proteins are crucial for genome organization and function.
• Architectural DNA-binding proteins bend DNA to facilitate various cellular processes.
• Understanding protein-DNA interactions is key to deciphering gene regulation and DNA repair.

Purpose of the Study:

• To investigate the binding preferences of prokaryotic HU and integration host factor (IHF), and eukaryotic HMG1 proteins.
• To examine how DNA flexibility, modulated by tandem mismatches and base modification (hmU), affects protein binding affinity.
• To elucidate the role of sequence-dependent DNA flexibility in target-site selection by these DNA-bending proteins.

Main Methods:

• Electrophoretic mobility shift assays (EMSAs) to determine binding affinities (Kd values).
• Synthesis of modified DNA substrates with tandem mismatches (loops) and 5-hydroxymethyluracil (hmU) substitution for thymine.
• Comparative analysis of protein binding to standard duplex DNA versus modified DNA structures.

Main Results:

• HU and IHF exhibit higher affinity for DNA containing 4-nucleotide loops compared to perfect duplex DNA.
• Optimal binding for HU and IHF is dependent on the separation and positioning of these loops.
• All three proteins (HU, IHF, HMG1) demonstrate increased binding affinity to DNA where thymine is replaced by 5-hydroxymethyluracil (hmU).
• IHF shows significant site-selectivity even in hmU-modified DNA, binding with higher affinity to its specific consensus site.

Conclusions:

• DNA flexibility, influenced by structural elements like loops and base modifications, significantly impacts the binding of architectural DNA-bending proteins.
• The substitution of thymine with hmU enhances DNA flexibility and protein-DNA interactions.
• Sequence-dependent variations in DNA flexibility play a critical role in the target-site selection mechanisms of HU, IHF, and HMG1.