Enzymic Unwinding of DNA

Abstract

The DNA-stimulated ATPase [EC 3.6.1.3; from Escherichia coli] is a DNA unwinding enzyme. Substrates employed were DNA.cntdot.RNA hybrid duplexes and DNA.cntdot.DNA partial duplexes prepared by polymerization on phage fd single-stranded DNA template. The enzyme denatured these duplexes in an ATP-dependent reaction without detectably degrading them. EDTA, an inhibitor of the Mg2+-requiring ATPase, prevented denaturation, suggesting that dephosphorylation of the ATP and not only its presence is required. The enzyme probably binds initially to a single-stranded part of the DNA substrate molecule and then, energetically supported by ATP dephosphorylation, it invades double-stranded parts separating base-paired strands by progressive, zipper-like action. Chain separation probably results from the combined action of several enzyme molecules, and the enzyme''s tendency to aggregate with itself reflects a molecular tendency to cooperate. The enzyme probably has many functions.