Specific fragmentation of T7 phage DNA at low-melting sites

DNA overstretching in the presence of glyoxal: structural evidence of force-induced DNA melting

When a long DNA molecule is stretched beyond its B-form contour length, a transition occurs in which its length increases by a factor of 1.7, with very little force increase. A quantitative model was proposed to describe this transition as force-induced melting, where double-stranded DNA is converted into single-stranded DNA. The force-induced melting model accurately describes the thermodynamics of DNA overstretching as a function of solution conditions and in the presence of DNA binding ligands. An alternative explanation suggests a transformation into S-DNA, a double-stranded form which preserves the interstrand base pairing. To determine the extent to which DNA base pairs are exposed to solution during the transition, we held DNA overstretched to different lengths within the transition in the presence of glyoxal. If overstretching involved strand separation, then force-melted basepairs would be glyoxal-modified, thus essentially permanently single-stranded. Subsequent stretches confirm that a significant fraction of the DNA melted by force is permanently melted. This result demonstrates that DNA overstretching is accompanied by a disruption of the DNA helical structure, including a loss of hydrogen bonding.

Thermal denaturation of double-stranded DNA: effect of base stacking

We study the thermal denaturation of double-stranded DNA, i.e., separation of its two strands upon heating. A simple homo-polymer model is used to account for the effect of base stacking on the thermal stability of DNA. We find that stacking influences the stability in a nontrivial way: It not only enhances the stability but also makes the denaturation transition sharp. While stacking between bound monomers stabilizes DNA as does base pairing, stacking in unbound parts (or loops) rather destabilizes DNA--the overall stability is, however, enhanced by stacking.

Single-strand-specific nucleases

Single-strand-specific nucleases, which act on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids, are multifunctional enzymes and are ubiquitous in distribution. They find wide application as analytical tools in molecular biology research, although enzymes such as P1 nuclease are also used for production of flavor enhancers such as 5' IMP and 5' GMP. Because these enzymes are mainly used as analytical tools, very little attention was paid to aspects relating to their structure-function relationships. However, during the last few years considerable developments have taken place in this area. Single-strand-specific nucleases, their purification, characteristics, biological role, and applications have been reviewed.

Localization of low-melting regions in phage T7 DNA

Specific fragmentation of T7 DNA at glyoxal-fixed denatured regions by the S1 endonuclease followed by restriction analysis made it possible to localize four low-melting regions in phage T7 DNA. These regions have the following coordinates:0.5-1.2;14.8+/-0.3;46.3+/-0.5; 98.4+/-0.3 (in T7 DNA length units). The location of the low-melting regions was refined by means of electron-microscopic denaturation mapping and gel electrophoresis of partially denatured DNA. The obtained localization of the low-melting regions is consistent with the available data on the sequence of T7 DNA. The map of low-melting regions was compared with the genetic map of T7 DNA.Images

UV-induced small structural changes in the T7 bacteriophage studied by melting methods

UV optical absorption and circular dichroism (CD) properties (spectra and melting curves) of T7 bacteriophage were investigated to detect "in situ" structural damages which can be related to the biological inactivation due to UV irradiation. UV doses (0.2-1.2 kJ/m2 at 254 nm) near to the biologically effective minimal dose were applied where the initial genetic damage (approximately 10 events/phage) was observed. The decrease of the melting temperature of the helix-coil transition and the broadening of the transition range indicate the destabilization of the intraphage structure due to the presence of about 0.1-0.6% damaged base concentration.

Gel electrophoresis of partially denatured DNA. Retardation effect: its analysis and application

The hypothesis about the role of partial denaturation in DNA retardation during its electrophoresis in denaturing gel /1,2/ was tested. We used partially melted DNA molecules in which the size of the melted regions and their location were known. They were obtained through glyoxal treatment of the melted regions by a procedure allowing the denatured state to be fixed at any point within the melting range. The approach and the availability of the melting maps of DNAs made it possible to investigate DNA molecules differing in length and in the size of the melted regions. The presence of a denatured region at the end of the molecule or inside of it was shown to decrease its electrophoretic mobility, the effect depending on the size of the melted region and on the DNA length. On the basis of the experimental results an explanation is proposed for the cause of retardation in the case of partially denatured DNA.

Equilibrium melting of plasmid ColE1 DNA: electron-microscopic visualization

The fine structure of the melting curve for the linear colE1 DNA has been obtained. To find the ColE1 DNA regions corresponding to peaks in the melting curve's fine structure, we fixed the melted DNA regions with glyoxal /12/. Electron-microscopic denaturation maps were obtained for nine temperature points within the melting range. Thereby the whole process of colE1 DNA melting was reconstructed in detail. Spectrophotometric and electron microscopic data were used for mapping the distribution of Gc-pairs over the DNA molecule. The most AT-rich DNA regions (28 and 37% of GC-pairs), 380 and 660 bp long resp., are located on both sides of the site of ColE1 DNA's cleavage by EcoR1 endonuclease. The equilibrium denaturation maps are compared with maps obtained by the method of Inman /20/ for eight points of the kinetic curve of ColE1 DNA unwinding by formaldehyde.

Measurement of the differential melting profile of a promoter-containing fragment of T7 DNA by means of a microspectrophotometer

A double-beam microspectrophotometer with a 5 microliter cell has been used to study denaturation of DNA in an aqueous solution. This instrument enables measurement of high-resolution differential melting profiles simultaneously at several wavelengths with 0.5 to 1 microgram of DNA. Therefore it becomes possible to study nucleic acids which are difficult to obtain in large amounts. The techniques have been employed to measure the differential melting profiles of T7 DNA and of a fragment of this DNA 1000 base pairs long which contains the four early promoters.