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Internal DNA pressure modifies stability of WT phage.

Irena Ivanovska¹, Gijs Wuite, Bengt Jönsson

¹Physics of Complex Systems, Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV, Amsterdam, The Netherlands.

Proceedings of the National Academy of Sciences of the United States of America

|May 31, 2007

Summary

Bacteriophage capsids with full-length DNA are twice as strong as those with shorter genomes. This increased strength, due to osmotic pressure from DNA-hydrating water, suggests shell strength limits genome size.

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Area of Science:

Structural biology
Biophysics
Molecular biology

Background:

Bacteriophages package double-stranded DNA (dsDNA) under high internal pressure.
Capsid strength is crucial for phage survival and genome delivery.

Purpose of the Study:

To investigate the correlation between packaged DNA length and bacteriophage capsid strength.
To understand the contribution of internal osmotic pressure to capsid mechanics.

Main Methods:

Atomic force microscope (AFM) indentation technique to measure capsid strength.
Analysis of lambda phage mutants with varying DNA lengths (78-100% of wild-type).
Development of an analytical model for DNA-filled capsid deformation.

Main Results:

Wild-type (WT) lambda phage capsids are twice as strong as shorter genome mutants.
Shorter genome mutants exhibit capsid strength similar to empty capsids.
Internal osmotic pressure, arising from DNA-hydrating water, increases exponentially with DNA density, significantly enhancing WT capsid strength.

Conclusions:

Capsid shell strength is a limiting factor for the maximum packaged genome length in bacteriophages.
The observed strength of WT phages is optimized to withstand external mechanical stresses in natural environments.
Evolutionary pressures likely favor genome sizes that balance packaging capacity with capsid integrity.