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Viruses 2017, 9(5), 119; doi:10.3390/v9050119

ATP-Driven Contraction of Phage T3 Capsids with DNA Incompletely Packaged In Vivo

Department of Biochemistry and Structural Biology, The University of Texas Health Science Center, San Antonio, TX 78229-3900, USA
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Author to whom correspondence should be addressed.
Academic Editor: Rob Lavigne
Received: 6 March 2017 / Revised: 1 May 2017 / Accepted: 10 May 2017 / Published: 19 May 2017
(This article belongs to the Section Bacterial Viruses)
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Abstract

Adenosine triphosphate (ATP) cleavage powers packaging of a double-stranded DNA (dsDNA) molecule in a pre-assembled capsid of phages that include T3. Several observations constitute a challenge to the conventional view that the shell of the capsid is energetically inert during packaging. Here, we test this challenge by analyzing the in vitro effects of ATP on the shells of capsids generated by DNA packaging in vivo. These capsids retain incompletely packaged DNA (ipDNA) and are called ipDNA-capsids; the ipDNA-capsids are assumed to be products of premature genome maturation-cleavage. They were isolated via preparative Nycodenz buoyant density centrifugation. For some ipDNA-capsids, Nycodenz impermeability increases hydration and generates density so low that shell hyper-expansion must exist to accommodate associated water. Electron microscopy (EM) confirmed hyper-expansion and low permeability and revealed that 3.0 mM magnesium ATP (physiological concentration) causes contraction of hyper-expanded, lowpermeability ipDNA-capsids to less than mature size; 5.0 mM magnesium ATP (border of supraphysiological concentration) or more disrupts them. Additionally, excess sodium ADP reverses 3.0 mM magnesium ATP-induced contraction and re-generates hyper-expansion. The Nycodenz impermeability implies assembly perfection that suggests selection for function in DNA packaging. These findings support the above challenge and can be explained via the assumption that T3 DNA packaging includes a back-up cycle of ATP-driven capsid contraction and hyper-expansion. View Full-Text
Keywords: agarose gel electrophoresis; bacteriophage assembly; biological motor; hydration-based buoyant density fractionation; electron microscopy agarose gel electrophoresis; bacteriophage assembly; biological motor; hydration-based buoyant density fractionation; electron microscopy
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Serwer, P.; Wright, E.T. ATP-Driven Contraction of Phage T3 Capsids with DNA Incompletely Packaged In Vivo. Viruses 2017, 9, 119.

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