Kinetoplast DNA maxicircles: networks within networks.

Kinetoplast DNA (kDNA), the mitochondrial DNA of trypanosomes, is an enormous network of interlocked minicircles and maxicircles. We selectively removed minicircles from Trypanosoma equiperdum kDNA networks by restriction enzyme cleavage. Maxicircles remained in aggregates that were resistant to protease or RNase and contained no residual minicircles, but were resolved into circular monomers by topoisomerase II. Maxicircles thus form independent catenanes within kDNA networks. Heterogeneity in the size, composition, and organization of maxicircle catenanes reflects changes that occur during kDNA replication. The rosette-like arrangement of maxicircle catenanes is distinctly different from that of minicircle catenanes. Trypanosome kDNA networks reveal unique topological complexity: they are composed of entirely dissimilar catenanes that are in turn extensively interlocked with one another.