Structural Studies of Bacteriophage T7 and T7 Capsids

Citation

Ruark, John Edward (1978) Structural Studies of Bacteriophage T7 and T7 Capsids. Master's thesis, California Institute of Technology. doi:10.7907/3nwd-0q45. https://resolver.caltech.edu/CaltechTHESIS:03302010-083049438

Abstract

The structures of bacteriophage T7 and T7 capsids have been investigated by small-angle X-ray scattering. Phage T7 has a radius of 301 ± 2 Å (excluding the phage tail), and a calculated volume of 1.14 ±.05 x 10^(-16) ml. The radius determined for. T7 phage in solution is about 30% greater than the radius measured from electron micrographs, indicating that considerable shrinkage and distortion occurs during sample preparation for electron microscopy. Thus the volume calculated from X-ray scattering in solution is almost twice that estimated from electron microscope measurements. Capsids that have a phage-like envelope and do not contain DNA were obtained from lysates of T7-infected E. coli (capsid II) and by separating the capsid component of T7 phage from the phage DNA using temperature shock (capsid IV). These capsids have outer radii of 2[8?]3-289 ± 2 Å and are thus significantly smaller than the envelope of T7 phage. The thickness of the envelopes of two forms of capsid II are 20 and 23 ± 2 Å. The volume in T7 phage available to package DNA is estimated to be 9.2 ± .4 x 10^(-17) ml or 2.1 times the volume of the B form of T7 DNA. A T7 precursor capsid (capsid I), believed to be capable of pulling in DNA, has a smaller outer diameter (261 ± 2 Å) and a thicker envelope (52 ± 4 Å) than capsids II and IV. Capsid I expands when pelleted in the ultracentrifuge, but expansion can be prevented by fixing with glutaraldehyde. A low resolution model for the capsid I envelope has been developed.

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