Enzyme-catalyzed DNA unwinding. The role of ATP in helicase III activity

The enzyme helicase III catalyzes ATP-dependent unwinding of double-stranded DNA (Yarranto, G. T., Das, R. H., and Gefter, M. L. (1979) J. Biol. Chem. 254, 11997-12001). The free enzyme is able to bind to double- and single-stranded DNA. In the presence of ATP the enzyme can bind single- but not double-stranded DNA. The enzyme catalyzes an ADP-ATP exchange reaction in the absence of DNA. It is suggested that there is an enzyme.phosphate complex that discriminates between the two forms of DNA. These results are discussed in relation to a model that accounts for catalytic unwinding of DNA coupled to ATP hydrolysis.