|
The double-stranded structure of DNA provides for an elegant mechanism to insure the accurate duplication and repair of DNA through the use of complementary base pairing rules. However, to make use of the properties of complementary base pairing the hydrogen bonds holding the double helix together must be disrupted, and the helix unwound, to expose single-stranded DNA templates for use by DNA polymerases, primases and other DNA modifying enzymes. This unwinding reaction is accomplished by a class of enzymes called DNA helicases. These enzymes disrupt the double helical structure of DNA in a nucleotide hydrolysis-dependent reaction. Examples of DNA helicase enzymes are now numerous in phage, bacteria, eukaryotic viruses, and eukaryotic cells indicating that these enzymes are ubiquitous. In addition, it is now clear that helicases are involved in all aspects of DNA metabolism including DNA replication, repair, recombination and transposition. Individual cells contain multiple DNA helicases and current data suggests each helicase has a unique biochemical role in the cell. It should be noted that resolution of RNA secondary structure is also a mechanistic requirement in essentially all aspects of RNA metabolism. Consequently, RNA helicases, with many similarities to the DNA helicases, also exist and are involved in all aspects of RNA metabolism.
The first DNA helicase isolated on the basis of its helicase activity was described in 1976 by Hartmut Hoffmann-Berling (Eur. Journal of Biochemistry 65:431). This protein, called DNA helicase I, catalyzed an ATP hydrolysis-dependent unwinding reaction. Since then dozens of proteins with helicase activity have been isolated and characterized. In addition, hundreds of putative DNA and RNA helicases have been identified in the various genome sequencing projects. Many of these enzymes await further biochemical and genetic characterization.
By studying many different helicases in multiple organisms it has become clear that helicases are involved in all aspects of DNA and RNA metabolism including (but not limited to) DNA replication, repair, recombination, transcription, pre-mRNA splicing, gene silencing, and translation. In addition, there are many helicases in each cell. For example, there are nearly a dozen DNA helicases and several RNA helicases in the prokaryote E. coli and the budding yeast S. cerevisiae has been estimated to contain more than 100 helicases. We know from biochemical and genetic studies in E. coli that each helicase is involved in a specific reaction pathway in the cell. In other words, one helicase does not directly substitute for another helicase in E. coli. It is not yet clear how much functional redundancy will be observed in eukaryotic cells.
In the last several years the crystal structure of several helicases has been solved; some in complexes with DNA molecules (for a review see Caruthers, J.M. and McKay, D.B. Current Opinion in Structural Biology (2002) 12:123). This has provided a very detailed view of these remarkable proteins and has provided our first real glimpse into how the energy supplied by ATP hydrolysis is coupled with the unwinding of duplex DNA.
The long-term goal of the laboratory is to understand the molecular role of several helicases in E. coli and S. cerevisiae. Use the links provided to learn about research goals and specific projects in the laboratory.
|
|
