Melting profiles of eight DNA molecules with lengths ranging from 849 to 4362 bp have been measured in an SSC buffer where the melting is an equilibrium process up to complete strand separation. A theoretical analysis shows that the melting profiles depend on only eight invariants that are linear combinations of 10 original stacking parameters. As a result it is impossible to determine the 10 parameters from the melting profiles. The 8 variants have been determined by fitting theoretical profiles to experimental ones for two fragments. Then theoretical and experimental profiles are compared for those 6 fragments that were not used in the fitting procedure. This comparison demonstrates that allowance for heterogeneous stacking considerably improves the agreement between theory and experiment. The values of invariants have proved to be small. This confirms the previous conclusion that heterogeneous stacking interactions produce only small corrections to the major effect of the difference in the mean stabilities of AT and GC pairs. Some discrepancy between theory and experiment that remains after the allowance for heterogeneous stacking is probably due to even finer effects of long-range interactions.

The paper presents measurements of the difference in the melting temperature of a colE1 DNA region when it is located inside the DNA helix and at its end. A direct comparison of calculations based on the rigorous theory of helix-coil transition with experimental data for .2 M Na+ (the conditions for fully reversible melting) yielded the value of 2.5-5x10(-5) for the cooperatively factor sigma. We discuss the reversibility of DNA melting and the possibility of applying the "all-or-nothing" concept to the melting of DNA regions.

The determination by Sanger et al. of the comple nucleotide sequence for phiX174 DNA has made it possible for the first time to compare directly theoretical and experimental DNA melting profiles. The comparison shows that the theory predicts the observed shape of the differential melting curve surprisingly well. Calculation of the denaturation maps allows the peaks on the curve to be correlated with cooperative melting out of concrete regions on the sequence of nucleotides.

The fine structure of the melting curve for the linear ColB1 DNA ha been obtained. To In the present study we fixed the melted regions with glyoxal to determine the ColEB1 regions whose melting corresponded to peaks in the fine structure of the melting curve.This method h"a earlier been used for localizing the AT-rich regions in T7 DNA revealed by the fine structure of the melting curves /12/. Glyoxal reacts with the amino-and iminogroups of bases and prevents complementary pairing; the product of its reaction with guanine is highly stable /13/.Glyoxal was used to fix the denatured regions appearing in DNA molecules at pre-chosen temperatures within the melting range. The set of denaturation maps subsequently constructed from electron micrographs made it possible to determine the size and location of the DNA regions corresponding to peaks in the fine structure of the melting curve. A combination of spectrophotometry and electron-microscopic visualization of the partially denatured molecules reveals the complete picture of the heat denaturation process. Using these results we construct'ed a map of the distribution of averaged GC-pair content over the ColEl DNA molecule.