Dynamic Properties of Nucleic Acids in Biosupramolecular Systems, as Studied by ³¹P NMR

Abstract

The dynamic properties of nucleic acids in five different types of intact supramolecular systems, namely, chicken erythrocyte chromatin, the wild type and a deletion mutant of the λ phage, lipid-containing phage PM2, and Alteromonas espejiana ribosomes, were investigated by means of ³¹P solid-state NMR. The nucleic acids in the different supramolecular systems showed unique dynamic properties, which are closely connected with their functions. The total anisotropy of the phosphorus chemical shift (Δσ=σ₃₃-σ₁₁) of the ribosomes was 210 ppm at 5°C. This anisotropy was much larger than those of any DNA complexes, suggesting the highly rigid structure of ribosomal RNA. In contrast, 160 ppm was the largest chemical shift anisotropy at 5°C for B-form DNA in the supramolecular systems. This flexibility would be essential for DNAs to exert their functions. The involvement of a condensation protein in the PM2 phage was supported by the chemical shift anisotropy. The spin-lattice relaxation time in the proton rotating frame [ T₁ρ(H)] of the nucleic acids became shorter with the increase in the effective field in the rotating frame for all systems examined, showing that the motions of the nucleic acids effective for the relaxation are in the slow motional regime or in the range of ω_Iτ_c=1 at 5°C. The motional state of DNA of the λ phage was found to change at about 20°C on the basis of the temperature dependence of the spin-lattice relaxation time of phosphorus (T₁). X-ray scattering experiments also showed a change in the organization of the λ phage particle at about 20°C. The dynamic state of the viral particle was suggested to play an important role in the infection mechanism of bacteriophages.