Conformational transitions in the Escherichia coli chromosome: Analysis by viscometry and sedimentation

The effects of physical, chemical and enzymatic treatments on purified folded chromosomes from Escherichia coli were monitored, by viscometry and sedimentation. The following chromosome conformational states were distinguishable: (a) folded, supertwisted; (b) ethidium bromides-relaxed; (c) partially unfolded by ribonuclease; and (d) completely unfolded. Viscometric analyses show that the chromosomal DNA is supertwisted and that this supertwisting is lost after ribonuclease digestion. Extensive incubation with pancreatic ribonuclease A or ribonuclease T₁ only partially unfolds the chromosomes; complete unfolding requires incubation at temperatures of 60 to 70°C, treatment with sodium dodecyl sulfate, or incubation with trypsin. The folded chromosomes were sensitive to trypsin attack only after the chromosomes were partially unfolded, suggesting that the proteins contributing to chromosome structure are buried inside the folded, supertwisted structure. It is now clear that both RNA and proteins are involved in maintaining the structure of the folded chromosome. Since ribonuclease and trypsin induced different conformational transitions, RNA and protein must stabilize different folds of the bacterial chromosome.