Sequence-dependent effects of spermine on the thermodynamics of the B-DNA to Z-DNA transition.
Spermine has been shown to bind to and stabilize a number of altered DNA conformations, including left-handed Z-DNA. Here, we have quantitatively studied the effects of spermine on the negative supercoil-induced transition from B- to Z-DNA. We have determined the intrinsic association constants for and the effective number of ligands that bind to both B- and Z-DNA. The intrinsic affinity of spermine for Z-DNA is approximately 10 times higher for d(CA/TG) (KZP = 1.2 x 10(8) M-1) than for d(CG) dinucleotides (KZP = 1.5 x 10(7) M-1), and both are greater than that for B-DNA (KBP = 1.4 x 10(5) M-1). This accounts for the stabilization of Z-DNA by spermine. The number of spermine accommodated by Z-DNA (nZ) is sequence-dependent [nZ = 0.6 spermine per 18 d(CA/TG) dinucleotides and 2.3 for 12 d(CG) dinucleotides]. The value of nZ of < 1 was interpreted as evidence for negative cooperativity in spermine binding to d(CA/TG) dinucleotides. Thus, although d(CA/TG) sequences saturate at lower spermine concentrations, the ligand has an overall greater effect on the stability of d(CG) dinucleotides as Z-DNA. B-DNA accommodates more spermines per base pair than either sequence as Z-DNA. At higher concentrations (> 10 microM), spermine destabilizes Z-DNA. Using these parameters in a model for competitive spermine binding to B-DNA and Z-DNA, we can make predictions for how potential Z-DNA sequences found in the human genome are affected by cellular levels of superhelical density and spermine.
Howell
M L
ML
Department of Biochemistry and Biophysics, Oregon State University, Corvallis 97331, USA.
Schroth
G P
GP
Ho
P S
PS
D004247:DNA / Q000737:chemistry / Q000378:metabolism*; D015180:Electrophoresis, Gel, Two-Dimensional; D006801:Humans; D009690:Nucleic Acid Conformation; D013096:Spermine / Q000494:pharmacology*; D013816:Thermodynamics
D013096:Spermine; D004247:DNA