Relaxation of Supercoiled DNA by DNA Modifying Agents: Detection by Gel Electrophoresis

Abstract

Figure 1 shows the electrophoretic pattern of different forms of the same circular DNA on agarose gel. In the left lane, the upper band represents the relaxed form (RFII) and the lower band the naturally negatively supercoiled form (RFI) of a plasmid DNA. The RFI band contains different topoisomers that can not be resolved by this technique. The right lane shows the electrophoretic pattern of supercoiled DNA that has been partially relaxed by the action of the topoisomerase I of Micrococcus luteus (an ω-like protein, Kung and Wang, 1977). A pattern of bands is formed from DNA molecules differing by ±1 linking number and by ∓l supertwisting of the axis of the helix. The reciprocal relationship between twisting of the strands of the double helix and supertwisting of the axis of the helix may be expressed in the quantitative form through the relationship:

$$L = T + W$$

where L, the linking number, is the number of times one DNA strand goes around the other; T is the twist of one strand about the other; W is the writhing number and describes the twisting of the axis of the helix upon itself (Crick, 1976).