For many aspects of DNA-protein interaction, it is vital to know how DNA bending rigidity (or persistence length, a) depends on its sequence. We addressed this problem using the method based on cyclization of short DNA fragments, which allows very accurate determination of a. Our approach was based on assigning specific values of a to each of 10 distinct dinucleotide steps. We prepared DNA fragments, each about 200 bp in length, with various quasiperiodic sequences, measured their cyclization efficiencies (j factors), and fitted the data by the theoretical equation to obtain the values of a for each fragment. From these data, we obtained a set of a for the dinucleotide steps. To test this set, we used it to design DNA sequences that should correspond to very low and very high values of a, prepared the corresponding fragments, and determined their values of a experimentally. The measured and calculated values of a were very close to one another, confirming that we have found the correct solution to this long-standing problem. The same experimental data also allowed us to determine the sequence dependence of DNA helical repeat.DNA elasticity | DNA persistence length T he value of DNA persistence length, a, closely related to the bending rigidity of the double helix, is very important for quantitative analysis of many aspects of DNA functioning. Many different methods were applied over the last decades to determine this value and its dependence on ionic conditions (1). These studies showed that under near physiological ionic conditions the value of a is close to 50 nm (1-3). They also showed that, in a good approximation, the value of a does not depend on DNA sequence, because many studies that use different DNA molecules gave very close values of a. However, the sequence dependence of DNA bending rigidity is vital for many aspects of DNAprotein interaction, and is particularly important for understanding how nucleosomes position along DNA molecules (4, 5). Despite its importance, the problem has formerly remained unsolved, although a few research groups have tried various approaches (6-8). The data that are generally considered most reliable were obtained from statistical analysis of DNA-protein crystal structures (9, 10). The large volume of available structural data allowed this group of researchers to present a very detailed picture of the sequence dependence of DNA conformational properties. This analysis, however, is based on the assumption that variations of the DNA bend angles in the crystals correspond to the amplitudes of thermal fluctuations of these angles in solution. It is hard to justify this assumption.The major obstacle in determining the sequence dependence of a is the necessity of highly accurate measurements of a for different sequences. The majority of methods applied for the measurements of a do not provide the necessary level of accuracy. The only known method that allows one to measure a with the required accuracy is based on cyclization of short DNA fragments by DNA ligase (2,3,11,12). It i...

We have determined the temperature dependence of DNA persistence length, a, using two different methods. The first approach was based on measuring the j-factors of short DNA fragments at various temperatures. Fitting the measured j-factors by the theoretical equation allowed us to obtain the values of a for temperatures between 5°C and 42°C. The second approach was based on measuring the equilibrium distribution of the linking number between the strands of circular DNA at different temperatures. The major contribution into the distribution variance comes from the fluctuations of DNA writhe in the nicked circular molecules which are specified by the value of a. The computation-based analysis of the measured variances was used to obtain the values of a for temperatures up to 60°C. We found a good agreement between the results obtained by these two methods. Our data show that DNA persistence length strongly depends on temperature and accounting for this dependence is important in quantitative comparison between experimental results obtained at different temperatures.

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A compact apparatus for transferring nonvolatile particles into the gas phase and depositing them on a solid surface has been built and tested successfully. As initial experiment, solid caffeine with a vanishingly low vapor pressure has been dissolved in supercritical carbon dioxide, expanded into vacuum using a pulsed, supersonic molecular beam, and detected using a simple residual gas analyzer.

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