DNA Topoisomerase Mutations in Bacteria
The chromosome of Escherichia coli is a DNA molecule having a length more than a thousand times that of the cell in which it resides (Cairns 1963). How this DNA is compacted and packaged is poorly understood. It appears that the DNA is arranged into about 50 large (100 kbp), topologically independent domains (Worcel and Burgi 1972; Sinden and Pettijohn 1981), and each of these domains, or loops, is probably under negative superhelical tension (Worcel and Burgi 1972; Sinden et al. 1980). While it is becoming increasingly clear that the superhelical tension is a result of topoisomerase action, how the loops are established and maintained is still a mystery. Another level of compaction appears to be the packaging of DNA into nucleosomelike structures (Griffith 1976; Varshavsky et al. 1977). Unfortunately, bacterial chromatin, unlike its eukaryotic counterpact, has been difficult to isolate and study.
KeywordsRecombination Titration Sedimentation Electrophoresis Fractionation
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