ANALYSIS OF P-31 NUCLEAR-MAGNETIC-RESONANCE LINESHAPES AND TRANSVERSAL RELAXATION OF BACTERIOPHAGE-M13 AND TOBACCO MOSAIC-VIRUS

The experimentally observed P-31 lineshapes and transversal relaxation of 15% (wt/wt) M13, 30% M13, and 30% tobacco mosaic virus (TMV) are c ompared with lineshapes and relaxation curves that are simulated for v arious types of rotational diffusion using the models discussed previo usly (Magusin, P. C. M. M., and M. A. Hemminga. 1993. Biophys. J. 64:1 851-1860). It is found that isotropic diffusion cannot explain the obs erved lineshape effects. A rigid rod diffusion model is only successfu l in describing the experimental data obtained for 15% M13. For 30% M1 3 the experimental lineshape and relaxation curve cannot be interprete d consistently and the TMV lineshape cannot even be simulated alone, i ndicating that the rigid rod diffusion model does not generally apply. A combined diffusion model with fast isolated motions of the encapsul ated nucleic acid dominating the lineshape and a slow overall rotation of the virion as a whole, which mainly is reflected in the transversa l relaxation, is able to provide a consistent picture for the 15 and 3 0% M13 samples, but not for TMV. Strongly improved lineshape fits for TMV are obtained assuming that there are three binding sites with diff erent mobilities. The presence of three binding sites is consistent wi th previous models of TMV. The best lineshapes are simulated for a com bination of one mobile and two static sites. Although less markedly, t he assumption that two fractions of DNA with different mobilities exis t within M13 also improves the simulated lineshapes. The possible exis tence of two P-31 fractions in M13 sheds new light on the nonintegral ratio 2.4:1 between the number of nucleotides and protein coat subunit s in the phage: 83% of the viral DNA is less mobile, suggesting that t he binding of the DNA molecule to the protein coat actually occurs at the integral ratio of two nucleotides per protein subunit.