Nonhistone proteins HMG1 and HMG2 unwind DNA double helix

In a previous communication we have shown that both HMG₁ and HMG₂ non-histone proteins change the DNA helical structure and the binding of HMG₁ and HMG₂ to DNA induces a net unwinding equivalent of DNA double helix (Javaherian, K., Liu, L. F. and Wang, J. C. (1978) Science, 199, 1345-1346). Employing melting absorption technique, we now show that in the presence of salt HMG₁ and HMG₂ destabilize DNA whereas in the absence of salt, they both stabilize DNA molecules. Consequently the folded structure of HMG must play an important role in melting DNA. Furthermore, by measuring topological winding number using competition unwinding experiments, we conclude that HMG₁ has a higher affinity for a single-stranded DNA relative to double-stranded DNA. These results together suggest that HMG₁ and HMG₂ unwind DNA double helix by local denaturation of the DNA base pairs. The net unwinding angles have been measured to be 22° and 26° per molecule of HMG₁ and HMG₂ respectively.