Abstract
Superhelical circular (form I) SV40 DNA was converted to linear molecules by the action of a partially purified restriction enzyme of Resistance Transfer Factor-R(1) of Escherichia coli. The resulting linear DNA molecules are full length, as judged by their sedimentation through alkaline sucrose gradient and by direct observation in an electron microscope. Nicked circular (form II) DNA was found as an intermediate in the conversion of form I DNA to linear DNA. Analysis of partial denaturation maps obtained by alkaline denaturation of the unitlength linear molecules showed that the break in SV40 DNA occurred at a specific site on the DNA.