The double-stranded structure of DNA provides for an elegant mechanism allowing the accurate duplication and repair of DNA. 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 an energy-requiring reaction that is dependent on the hydrolysis of nucleoside 5′-triphosphates. Examples of DNA helicase enzymes are now numerous in phage, bacteria, eukaryotic viruses, and in eukaryotic cells suggesting that these enzymes are ubiquitous. In addition, we now know that helicases are involved in all aspects of DNA metabolism. Individual cells contain multiple DNA helicases; each helicase apparently has a unique biochemical role in the cell.
For the last several years we have focused our studies on the enzymatic mechanisms and biological roles of DNA helicases. The bacterium E. coli and the budding yeast S. cerevisiae provide attractive systems in which to pursue these studies due to the ease of genetic manipulation and the ability to isolate enzymes for biochemical studies. A large body of literature now exists for the enzymes found in E. coli. Yeast, however, remains largely unexplored. We are directing our efforts toward characterizing DNA helicases from both E. coli and yeast. The long-range goal of the research program is to understand, in enzymatic and molecular terms, the mechanism of action of several helicase enzymes, and to define their individual roles in DNA metabolism.
The lab also has an interest in the process of DNA transfer by bacterial conjugation, defined as the unidirectional and horizontal transmission of genetic information between bacterial cells. Conjugation was discovered more than 50 years ago yet we still have only a rudimentary knowledge of the molecular details surrounding this important mechanism for DNA transfer. Previous studies have shown that DNA transfer begins at a site- and strand-specific nick in the conjugative plasmid. The double-stranded DNA molecule is then unwound from this nick, presumably by a DNA helicase, and a single-strand of DNA is transferred into the recipient cell. This laboratory has demonstrated, using the F plasmid as a paradigm, the participation of two F plasmid-encoded proteins, TraIp and TraYp, and one host-encoded protein, integration host factor (IHF), in the nicking reaction; subsequent unwinding has not yet been reconstituted in any system. The goal of this project is to arrive at a more detailed understanding of this key process.