SynopsisQuasielastic and static light-scattering measurements were made on DNA isolated from chicken erythrocyte mononucleosomes as a function of ionic strength between 6 X and 1.OM. A transition from single-exponential autocorrelation functions to markedly non-single-exponential decays was observed around 10-2M ionic strength and was accompanied by a large decrease in the excess light-scattering intensity. Autocorrelation functions recorded below 10-2M salt were well fit by the sum of two exponential relaxation which differed by as much as 100-fold in time constants. Apparent diffusion coefficients for the fast and slow processes plateaued around 10-3M with numerical values approximately 10-fold and l/Lo, respectively, of the translational diffusion coefficient for mononucleosome DNA at high ionic strength. This behavior is similar to that observed with poly(L-lysine), for which the slow decay has been associated with a transition to an extraordinary phase. The strong and complex salt dependence ohserved here illustrates potential difficulties in deriving structural information from scattering by polyions at low ionic strength.
The gene 1 protein of Salmonella bacteriophage P22 is located at the DNA packaging vertex of the mature particle. The protein is incorporated into the procapsid shell during shell assembly and is required for DNA packaging. The unassembled precursor form of the gene 1 protein has been purified from cells infected with mutants blocked in procapsid assembly. The purified 90,000-dalton protein was dimeric or monomeric; upon storage in the cold it formed 20S cyclic dodecamers. Computer filtering of negatively stained electron micrographs revealed 12 arms and knobs projecting from a central ring, with a 30-A channel at the center. Similar dodecameric rings were released from disrupted procapsid shells. These results indicate that the gene 1 protein is organized as a cyclic dodecamer within the procapsid shell and serves as the portal through which P22 DNA is threaded during DNA packaging. The presence of a 12-fold ring located at a 5-fold portal vertex appears to be a conserved structural theme of the DNA packaging apparatus of double-stranded DNA phages.
Bacteriophage phi W14 DNA carries the hypermodified, positively charged (2+) base alpha-putrescinylthymine (puThy) and consequently exhibits a decreased average linear charge density compared to the conventional B-form DNA helix. Noting that the unusual physical characteristics may contribute to the collapse properties of this DNA and facilitate the exceptionally high density of packaging of its genome in phi W14, I used total intensity light scattering to determine in vitro the critical concentrations of spermidine (Spd, 3+) required to induce the cooperative, monomolecular collapse of wild-type and mutant phi W14 DNA samples and quasi-elastic light scattering to compare the dynamic characteristics of the compacted particles. The DNA samples carried various percentages of the modified base with average charge spacings ranging from 1.3 to 2.2 A in comparison to T4 phage DNA (1.7 A). The results are analyzed and discussed both from a general theoretical point of view according to the counterion condensation theory of Manning [Manning, G. S. (1978) Q. Rev. Biophys. 11, 179-246] and from the more specialized aspect of DNA packaging in phi W14. In accord with theory, DNAs of lower charge density require a considerably higher critical counterion concentration (up to 118 microM Spd), whereas the outside diameter of the toroidal condensates, which they form, varies only marginally. Specific ion effects were probed by substituting hexaamminecobalt(III) (Hc, 3+) for Spd. Hc appears to be more efficient than Spd: it induces the collapse of all DNA samples at only one-sixth the critical concentration of Spd, and its condensates are 30% smaller (1072-1142 A vs. 744-800 A) except for wild-type phi W14 DNA, which forms Hc-collapsed particles indistinguishable from Spd-induced condensates. Collapse occurs, again with the exception of wild-type phi W14 DNA, when approximately 89% of the charges on each DNA are neutralized by territorially bound Spd. I conclude that the driving force for condensation clearly is a function of the charge density of the DNA and that the charge distribution may be an important factor in determining the degree of neutralization at which collapse becomes possible. The sample with the lowest charge density, wild-type phi W14 DNA, does not follow the trends set by the other members of the series.(ABSTRACT TRUNCATED AT 400 WORDS)
This paper considers the effect of chemical reactions on the spectrum of light scattered from macromolecular solutions. A general formalism is developed, taking into account only diffusion and chemical reaction, using the matrix eigenvalue technique of Salsburg and coworkers. This formalism is applied to the reactions A ^B and 2A
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