Electron microscopy of the circular kinetoplastic DNA from Trypanosoma cruzi: occurrence of catenated forms.

Circular kinetoplastic DNA was fractionated from Trypanosoma cruzi DNA by Hg-Cs(2)SO(4) gradient. Covalantly closed circular DNA was isolated by buoyant density centrifugation in ethidium bromide CsCl density gradients. Electron microscope studies have shown that kinetoplastic DNA molecules are in the form of open and closed circles of contour length 0.45 mu, the smallest known microbial DNA. DNA molecules are also in the form of catenanes consisting of two or more topologically interlocked circular units of the monomer size 0.45 mu. Associations of numerous circles have also been described and confirmed by band sedimentation. Long linear DNA molecules were also seen, their size ranging between 2 and 12 mu. These molecules are free or are attached to associations of circles.