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JBC, Vol. 252, Issue 17, 6208-6216, Sep, 1977
P. T. Englund, D. C. DiMaio and S. S. Price
The mitochondrial DNA of the protozoan Leishmania tarentolae, known as
kinetoplast DNA, contains thousands of minicircles linked in a
two-dimensional network. When kinetoplast DNA from exponentially growing
cells is centrifuged to equilibrium in a CsCl/ethidium bromide gradient, it
is resolved into two discrete components, Form I and Form II. Nearly all of
the minicircles in Form I networks are covalently closed and all of those
in Form II networks are open. These forms are indistinguishable from each
other when examined by electron microscopy and they appear identical when
analyzed by gel electrophoresis after digestion with the restriction
enzymes Hae III or Hpa II. However, Form II networks sediment roughly 50%
faster than Form I networks on a neutral sucrose gradient, indicating that
Form II networks are larger in size or more compact in conformation, or
both. Analysis of denatured Form II DNA by sedimentation or electron
microscopy indicates that nearly all of its minicircles have one or more
interruptions in both strands. Since the majority of the Form II
minicircles can be closed by DNA ligase, most of these interruptions must
be nicks. Experiments with S1 nuclease indicate that some small gaps may
also exist in Form II minicircles. 5'-Terminal nucleotide analysis of Form
II kinetoplast DNA does not suggest that the interruptions are at specific
locations in the minicircles. The significance of the two forms of
kinetoplast DNA has not yet been determined, but it is possible that Form
II is an intermediate in replication of this DNA.
A nicked form of kinetoplast DNA in Leishmania tarentolae
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