dsDNA in bacteriophages is highly stressed and exerts internal pressures of many atmospheres (1 atm ‫؍‬ 101.3 kPa) on the capsid walls. We investigate the correlation between packaged DNA length in phage (78 -100% of WT DNA) and capsid strength by using an atomic force microscope indentation technique. We show that phages with WT DNA are twice as strong as shorter genome mutants, which behave like empty capsids, regardless of high internal pressure. Our analytical model of DNA-filled capsid deformation shows that, because of DNA-hydrating water molecules, an osmotic pressure exists inside capsids that increases exponentially when the packaged DNA density is close to WT phage. This osmotic pressure raises the WT capsid strength and is approximately equal to the maximum breaking force of empty shells. This result suggests that the strength of the shells limits the maximal packaged genome length. Moreover, it implies an evolutionary optimization of WT phages allowing them to survive greater external mechanical stresses in nature.atomic force microscopy ͉ viral capsids ͉ bacteriophage ͉ DNA hydration forces ͉ osmotic pressure T he majority of viruses have spherical protein shells (capsids) with icosahedral symmetry, with radii varying between 10 and 100 nm, and with thicknesses of few nanometers, corresponding to a single protein layer. Viral capsids protect genomes that can be tens of micrometers in contour length. In prokaryotic viruses (bacteriophages), capsid proteins first assemble in empty capsids before the genome is actively packaged by a molecular motor that is part of the capsid (1, 2). Matching the capsid size and genome length is of great importance for efficient packaging and viral infectivity. For example, WT phage infects Escherichia coli cells and has its DNA (48,502 bp) contained in an icosahedral T ϭ 7 capsid to which a flexible, noncontractile tail is attached. The mature capsid has an outer diameter of 63 nm and a shell thickness of between 1.8 and 4.1 nm (1, 2). phages can be packaged with DNA lengths in the range of 78-106% of WT DNA and remain infectious (3). If the genome is shorter than 78% of the WT-DNA, then the phage fails to infect. When the DNA is longer than 106% of WT length, packaging does not occur. It was recently shown that dsDNA inside many phages is highly stressed because of electrostatic repulsion and the bending energy of the packaged DNA chain, resulting in internal pressures of several tens of atmospheres (1 atm ϭ 101.3 kPa) (4-13). This finding suggests that the infection is in part driven by the internal DNA pressure. Thus, if DNA is significantly shorter than WT, the internal pressure becomes too low and is therefore incapable of injecting enough DNA into the bacteria. On the other hand, if the DNA is longer than WT, the internal pressure might be too high, and the force that builds up in the capsid could exceed either the strength of the packaging motor (4, 14) or the maximum internal force that the capsid can withstand. Thus, the capsid size and strength might limit th...

shows the indentation rate of 60 nm/s, at which the DNA relaxation rate during the indentation is faster than the AFM tip indentation rate (i.e., indentation occurs at equilibrium). The gray dashed line shows the indentation rate corresponding to the DNA sliding/ejection rate of 60,000 bp/s determined in ref. 9.

No abstract

This work addresses the problem of determining micellar aggregation numbers for dilute ionic surfactant systems by means of the time-resolved fluorescence quenching method. We argue that the use of quenchers that are themselves surfactants gives us two advantages. First, the altering of the micelles caused by the solubilization of quencher molecules is minimized. Second, the distribution of the quencher between the micelles and the aqueous subphase can be obtained. The latter point is particularly important for the case of dilute micellar systems and when the micelles are adsorbed at interfaces or associated with polymers. We describe a method to obtain the partitioning of the quencher for various surfactant/quencher combinations. The method is based on a detailed thermodynamic model of mixed micelles supported by Poisson−Boltzmann cell model calculations. It is shown that ideal mixing of surfactant and quencher in the micelles simplifies the analysis of effects related to polydispersity and probe distribution among the micelles. The method is applied to quaternary ammonium surfactants, both mono- and divalent, with various chain lengths, using the corresponding alkyl pyridinium ions as quenchers. Aggregation numbers at concentrations close to the critical micelle concentration (cmc) are presented and discussed.