Salt‐concentration dependence of melting profiles of lambda phage DNAs: evidence for long‐range interactions and pronounced end effects

Abstract Melting profiles of DNAs from wild‐type λ phage and a deletion mutant phage λb2 were examined in a wide range of salt concentration. The fine structure of the melting profiles changed sharply with salt concentration, especially in the range [Na + ] ⩽ 10 m M . A comparison of the melting profiles between the wild‐type and the deletion mutant DNAs provided good evidence for extremely high melting cooperativity under low salt conditions, which is clearly manifested as the long‐range interactions and the pronounced end effects; a large melting peak appeared as a result of the b2 deletion without any inserted sequence in the salt range [Na + ] ⩽ 2.8 m M . It was also suggested that in the further reduced salt range [Na + ] ⩽ 2.0 m M , melting of a λ DNA molecule starts from its right end rather than the most (A + T)‐rich central region. The molecular basis of the high melting cooperativity at low salt concentrations can be explained in terms of the increased free energy associated with loop formation in the double‐helical structure of DNA.