Structure of Packaged DNA

Information of the latter kind was obtained by cryo-EM of tailless T7 heads, which were serendipitously found to present two preferential 

How many shells are there in the spool The number of rings seen in the averaged axial projection is 11 (Fig. 9e), whereas the number of shells required to accommodate the T7 genome, given geometrically 

The conformation of encapsidated T7 DNA has been studied by Raman difference spectroscopy and compared with free DNA at an approximately 10-fold lower density (Overman et al, 1998). The results, essentially recapitulating an earlier study of P22 DNA, which is of similar size and packing density (Aubrey et al, 1992), were that the DNA is in the conventional B-form and that kinking, if any, is limited by an upper bound of 2% of bases. The only difference detected was in two spectral bands associated with phosphates along the rim of the helices (Overman et al, 1998). This perturbation could be simulated in solution by increasing the concentration of Mg ions, thus it was suggested that T7 heads may also contain magnesium in appropriate concentrations. 

We thank Dr. D. M. Belnap (LSBR) for help with the procapsid handedness determination experiment Dr. E. Kocsis (OD-NIH) for help with the ROTASTAT program Drs. J.-M. Valpuesta and J. L. Carrascosa (Centro de Biotecnologia Molecular, Madrid) for kindly providing the images for Fig. 2g, j, and k and Dr. R. Wade (IBS) for providing computational facilities. 

Burnett, and R. L. Garcea, Eds.), pp. 312-351. Oxford University Press, New York. Steven, A. C., and Trus, B. L. (1986). In Electron Microscopy of Proteins, Vol. 5 Viral Structure . Academic Press, London. 

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