In this essay, we consider helicases, defined as enzymes that use the free energies of binding and hydrolysis of ATP to drive the unwinding of double-stranded nucleic acids, and ask how they function within, and are "coupled" to, the macromolecular machines of gene expression. To illustrate the principles of the integration of helicases into such machines, we consider the macromolecular complexes that direct and control DNA replication and DNA-dependent RNA transcription, and use these systems to illustrate how machines centered around coupled polymerase-helicase systems can be regulated by small changes in the interactions of their functional components.
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