Pf1 bacteriophage replication-assembly complex: X-ray fibre diffraction and scanning transmission electron microscopy |
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Authors: | G.G. Kneale R. Freeman D.A. Marvin |
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Affiliation: | European Molecular Biology Laboratory Heidelberg, Federal Republic of Germany |
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Abstract: | X-ray fibre diffraction and scanning transmission electron microscopy have been used to investigate the structure of an intracellular complex between circular single-stranded viral DNA and a viral DNA-binding protein. This complex is an intermediate between replication and assembly of the filamentous bacteriophage Pf1. By scanning transmission electron microscopy, the complex has a length of 1.00 μm and Mr = 29.6 × 106. It consists of 1770 protein subunits, each of 15,400 Mr, and one viral DNA molecule of 2.3 × 106Mr: there are 4.2 ± 0.5 nucleotides per subunit. The structure is flexible in solution, but in oriented dry fibres it forms a regular helix of 45 Å pitch having 6.0 dimeric protein subunits per turn, with an axial spacing of 7.5 Å between dimers and 1.9 Å between adjacent nucleotides. Model calculations suggest that the protein dimers may be oriented in a direction approximately perpendicular to the 45 Å helix, so that each dimer spans the two anti-parallel DNA chains. The results imply that conformational changes are required of the DNA as it is transferred from the double-stranded form to the replication-assembly complex, and subsequently to the virion. |
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