Structural investigations of DNA-histone complexes. A spin label study

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Histone H1 plays a crucial role in DNA packaging and chromatin structure.
• Understanding histone-DNA interactions is essential for deciphering gene regulation.
• Acridine derivatives serve as valuable probes for studying DNA-ligand interactions.

Purpose of the Study:

• To investigate the binding mechanism of histone H1 to DNA using novel acridine spin labels.
• To determine the specific binding sites and modes of histone H1 on DNA.
• To elucidate the influence of histone H1 on DNA intercalation.

Main Methods:

• Synthesis of two acridine spin labels: 6-chloro-9-[4-(2,2,6,6-tetramethyl-1-piperidinyloxy)amino]-2-methoxyacridine (I) and 9-[4-(2,2,6,6-tetramethyl-1-piperidinyloxy)amino]-acridine (II).
• Electron spin resonance (ESR) spectroscopy to analyze the immobilization and hyperfine splitting (2T11) of spin labels upon binding to DNA, polydA-polydT, and polydG-polydC.
• Competition assays with varying concentrations of histone H1 (P/D ratios) and salt (NaCl) to assess displacement of spin labels from DNA.

Main Results:

• ESR spectra indicated highly immobilized radicals for both spin labels (I and II) when bound to DNA and synthetic polynucleotides, with distinct hyperfine splitting values.
• Histone H1 addition at low P/D released both spin labels from DNA.
• In the presence of 0.1 M NaCl, histone H1 significantly released spin label II but not I, suggesting differential binding affinities.
• At high P/D ratios, histone H1 did not displace either spin label, irrespective of salt concentration.

Conclusions:

• Histone H1 binding to DNA does not appear to disrupt acridine intercalation sites.
• Histone H1 likely binds to DNA grooves, predominantly the major groove.
• Histone H1 exhibits preferential binding to A-T-rich regions of DNA.