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Nucleic Acids Research, 1978, Vol. 5, No. 12 4819-4836
© 1978

Articles

The helix-coil transition of closed and nicked DNAs in aqueous neutral trichloroacetate solutions

Rae Lyn Burke{dagger} and William R. Bauer

Department of Microbiology, Health Sciences Center, State University of New York at Stony Brook Stony Brook, NY 11794, USA

Received October 3, 1978.

The melting transition for closed, underwound DNAs and for nicked or linear DNAs was monitored by velocity sedimentation and by absorbance spectroscopy in aqueous NaCCl3CO2 (NaTCA) and RbTCA. The addition of neutral trichioroacetate lowers the midpoint of the helix-coil transition by 26°C/M for RbTCA and by 32°C/M for NaTCA, depressing the denaturation region to near room temperature at neutral pH. The melting of nicked DNA is cooperative, occurring over a temperature range of about 5.6°C. The melting profile for closed DNA is broad and noncooperative with a transition breadth greater than 45°. Closed DNAs undergo a structural alteration, as revealed by velocity sedimentation, resulting in a reduction in the number of superhelical turns at temperatures and salt concentrations substantially below the melting temperature of the nicked DNA. Tha reduction in the extent of supercoiling continues upon isothermal addition of salt up to the salt concentration at which all superhelical turns are removed. The salt concentration at the principal minimum in the sedimentation velocity profile (3.16 M NaTCA for PM-2 DNA) is approximately the same as that at the midpoint of the helix-coil transition for the nicked DNA.

{dagger} Present Address: Department of Biochemistry and Biophysics University of California Medical Center San Francisco, California 94147


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