Helicase mechanisms and the coupling of helicases within macromolecular machines. Part II: Integration of helicases into cellular processes.

In Part I of this review [Delagoutte & von Hippel, Quarterly Reviews of Biophysics (2002) 35, 431-478] we summarized what is known about the properties, mechanisms, and structures of the various helicases that catalyze the unwinding of double-stranded nucleic acids. Here, in Part II, we consider these helicases as tightly integrated (or coupled) components of the various macromolecular machines within which they operate. The biological processes that are considered explicitly include DNA replication, recombination, and nucleotide excision repair, as well as RNA transcription and splicing. We discuss the activities of the constituent helicases (and their protein partners) in the assembly (or loading) of the relevant complex onto (and into) the specific nucleic acid sites at which the actions of the helicase-containing complexes are to be initiated, the mechanisms by which the helicases (and the complexes) translocate along the nucleic acids in discharging their functions, and the reactions that are used to terminate the translocation of the helicase-containing complexes at specific sites within the nucleic acid 'substrate'. We emerge with several specific descriptions of how helicases function within the above processes of genetic expression which, we hope, can serve as paradigms for considering how helicases may also be coupled and function within other macromolecular machines.