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Abstract

DNA helicases catalyze the separation of double-stranded DNA into single strands using the energy of ATP hydrolysis. The four helicases which have been found in Escherichia coli are listed in Table 1. These enzymes are the helicases I, II, III and the helicase specified by the rep gene. As the table shows the four proteins differ considerably with respect to Mr, number of molecules per cell and, in particular, mechanism of action. Each of the helicases requires a region of single-stranded DNA to initiate unwinding, and each of the four enzymes unwinds DNA unidirectionally relative to the chemical polarity of the DNA strand to which it is bound. The direction of unwinding depends on the nature of the helicase.